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Basic units of chromatin structure

A nucleosome is the basic structural unit of DNA packaging in eukaryotes. The structure of a nucleosome consists of a segment of DNA wound around eight histone proteins[1] and resembles thread wrapped around a spool. The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4.

DNA must be compacted into nucleosomes to fit within the cell nucleus.[2] In addition to nucleosome wrapping, eukaryotic chromatin is further compacted by being folded into a series of more complex structures, eventually forming a chromosome. Each human cell contains about 30 million nucleosomes.[3]

Nucleosomes are thought to carry epigenetically inherited information in the form of covalent modifications of their core histones. Nucleosome positions in the genome are not random, and it is important to know where each nucleosome is located because this determines the accessibility of the DNA to regulatory proteins.[4]

Nucleosomes were first observed as particles in the electron microscope by Don and Ada Olins in 1974,[5] and their existence and structure (as histone octamers surrounded by approximately 200 base pairs of DNA) were proposed by Roger Kornberg.[6][7] The role of the nucleosome as a regulator of transcription was demonstrated by Lorch et al. in vitro[8] in 1987 and by Han and Grunstein[9] and Clark-Adams et al.[10] in vivo in 1988.

The nucleosome core particle consists of approximately 146 base pairs (bp) of DNA[11] wrapped in 1.67 left-handed superhelical turns around a histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4.[12] Core particles are connected by stretches of linker DNA, which can be up to about 80 bp long. Technically, a nucleosome is defined as the core particle plus one of these linker regions; however the word is often synonymous with the core particle.[13] Genome-wide nucleosome positioning maps are now available for many model organisms and human cells.[14]

Linker histones such as H1 and its isoforms are involved in chromatin compaction and sit at the base of the nucleosome near the DNA entry and exit binding to the linker region of the DNA.[15] Non-condensed nucleosomes without the linker histone resemble "beads on a string of DNA" under an electron microscope.[16]

In contrast to most eukaryotic cells, mature sperm cells largely use protamines to package their genomic DNA, most likely to achieve an even higher packaging ratio.[17] Histone equivalents and a simplified chromatin structure have also been found in Archaea,[18] suggesting that eukaryotes are not the only organisms that use nucleosomes.

Structure

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Structure of the core particle

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The crystal structure of the nucleosome core particle consisting of H2A , H2B , H3 and H4 core histones, and DNA. The view is from the top through the superhelical axis.

Overview

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Pioneering structural studies in the 1980s by Aaron Klug's group provided the first evidence that an octamer of histone proteins wraps DNA around itself in about 1.7 turns of a left-handed superhelix.[19] In 1997 the first near atomic resolution crystal structure of the nucleosome was solved by the Richmond group at the ETH Zurich, showing the most important details of the particle. The human alpha satellite palindromic DNA critical to achieving the 1997 nucleosome crystal structure was developed by the Bunick group at Oak Ridge National Laboratory in Tennessee.[20][21][22][23][24] The structures of over 20 different nucleosome core particles have been solved to date,[25] including those containing histone variants and histones from different species. The structure of the nucleosome core particle is remarkably conserved, and even a change of over 100 residues between frog and yeast histones results in electron density maps with an overall root mean square deviation of only 1.6Å.[26]

The nucleosome core particle (NCP)

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The nucleosome core particle (shown in the figure) consists of about 146 base pair of DNA[11] wrapped in 1.67 left-handed superhelical turns around the histone octamer, consisting of 2 copies each of the core histones H2A, H2B, H3, and H4. Adjacent nucleosomes are joined by a stretch of free DNA termed linker DNA (which varies from 10 - 80 bp in length depending on species and tissue type[18]).The whole structure generates a cylinder of diameter 11 nm and a height of 5.5 nm.

Apoptotic DNA laddering. Digested chromatin is in the first lane; the second contains DNA standard to compare lengths.
Scheme of nucleosome organization[27]
The crystal structure of the nucleosome core particle (PDB: 1EQZ​[28])

Nucleosome core particles are observed when chromatin in interphase is treated to cause the chromatin to unfold partially. The resulting image, via an electron microscope, is "beads on a string". The string is the DNA, while each bead in the nucleosome is a core particle. The nucleosome core particle is composed of DNA and histone proteins.[29]

Partial DNAse digestion of chromatin reveals its nucleosome structure. Because DNA portions of nucleosome core particles are less accessible for DNAse than linking sections, DNA gets digested into fragments of lengths equal to multiplicity of distance between nucleosomes (180, 360, 540 base pairs etc.). Hence a very characteristic pattern similar to a ladder is visible during gel electrophoresis of that DNA.[27] Such digestion can occur also under natural conditions during apoptosis ("cell suicide" or programmed cell death), because autodestruction of DNA typically is its role.[30]

Protein interactions within the nucleosome
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The core histone proteins contains a characteristic structural motif termed the "histone fold", which consists of three alpha-helices (α1-3) separated by two loops (L1-2). In solution, the histones form H2A-H2B heterodimers and H3-H4 heterotetramers. Histones dimerise about their long α2 helices in an anti-parallel orientation, and, in the case of H3 and H4, two such dimers form a 4-helix bundle stabilised by extensive H3-H3' interaction. The H2A/H2B dimer binds onto the H3/H4 tetramer due to interactions between H4 and H2B, which include the formation of a hydrophobic cluster.[12] The histone octamer is formed by a central H3/H4 tetramer sandwiched between two H2A/H2B dimers. Due to the highly basic charge of all four core histones, the histone octamer is stable only in the presence of DNA or very high salt concentrations.

Histone - DNA interactions
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The nucleosome contains over 120 direct protein-DNA interactions and several hundred water-mediated ones.[31] Direct protein - DNA interactions are not spread evenly about the octamer surface but rather located at discrete sites. These are due to the formation of two types of DNA binding sites within the octamer; the α1α1 site, which uses the α1 helix from two adjacent histones, and the L1L2 site formed by the L1 and L2 loops. Salt links and hydrogen bonding between both side-chain basic and hydroxyl groups and main-chain amides with the DNA backbone phosphates form the bulk of interactions with the DNA. This is important, given that the ubiquitous distribution of nucleosomes along genomes requires it to be a non-sequence-specific DNA-binding factor. Although nucleosomes tend to prefer some DNA sequences over others,[32] they are capable of binding practically to any sequence, which is thought to be due to the flexibility in the formation of these water-mediated interactions. In addition, non-polar interactions are made between protein side-chains and the deoxyribose groups, and an arginine side-chain intercalates into the DNA minor groove at all 14 sites where it faces the octamer surface. The distribution and strength of DNA-binding sites about the octamer surface distorts the DNA within the nucleosome core. The DNA is non-uniformly bent and also contains twist defects. The twist of free B-form DNA in solution is 10.5 bp per turn. However, the overall twist of nucleosomal DNA is only 10.2 bp per turn, varying from a value of 9.4 to 10.9 bp per turn.

Histone tail domains

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The histone tail extensions constitute up to 30% by mass of histones, but are not visible in the crystal structures of nucleosomes due to their high intrinsic flexibility, and have been thought to be largely unstructured.[33] The N-terminal tails of histones H3 and H2B pass through a channel formed by the minor grooves of the two DNA strands, protruding from the DNA every 20 bp. The N-terminal tail of histone H4, on the other hand, has a region of highly basic amino acids (16–25), which, in the crystal structure, forms an interaction with the highly acidic surface region of a H2A-H2B dimer of another nucleosome, being potentially relevant for the higher-order structure of nucleosomes. This interaction is thought to occur under physiological conditions also, and suggests that acetylation of the H4 tail distorts the higher-order structure of chromatin.[citation needed]

Higher order structure

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The current chromatin compaction model

The organization of the DNA that is achieved by the nucleosome cannot fully explain the packaging of DNA observed in the cell nucleus. Further compaction of chromatin into the cell nucleus is necessary, but it is not yet well understood. The current understanding[25] is that repeating nucleosomes with intervening "linker" DNA form a 10-nm-fiber, described as "beads on a string", and have a packing ratio of about five to ten.[18] A chain of nucleosomes can be arranged in a 30 nm fiber, a compacted structure with a packing ratio of ~50[18] and whose formation is dependent on the presence of the H1 histone.

A crystal structure of a tetranucleosome has been presented and used to build up a proposed structure of the 30 nm fiber as a two-start helix.[34] There is still a certain amount of contention regarding this model, as it is incompatible with recent electron microscopy data.[35] Beyond this, the structure of chromatin is poorly understood, but it is classically suggested that the 30 nm fiber is arranged into loops along a central protein scaffold to form transcriptionally active euchromatin. Further compaction leads to transcriptionally inactive heterochromatin.

Dynamics

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Although the nucleosome is a very stable protein-DNA complex, it is not static and has been shown to undergo a number of different structural re-arrangements including nucleosome sliding and DNA site exposure. Depending on the context, nucleosomes can inhibit or facilitate transcription factor binding. Nucleosome positions are controlled by three major contributions: First, the intrinsic binding affinity of the histone octamer depends on the DNA sequence. Second, the nucleosome can be displaced or recruited by the competitive or cooperative binding of other protein factors. Third, the nucleosome may be actively translocated by ATP-dependent remodeling complexes.[36]

Nucleosome sliding

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When incubated thermally, nucleosomes reconstituted onto the 5S DNA positioning sequence were able to reposition themselves translationally onto adjacent sequences.[37] This repositioning does not require disruption of the histone octamer but is consistent with nucleosomes being able to "slide" along the DNA in cis. CTCF binding sites act as nucleosome positioning anchors so that, when used to align various genomic signals, multiple flanking nucleosomes can be readily identified.[38] Although nucleosomes are intrinsically mobile, eukaryotes have evolved a large family of ATP-dependent chromatin remodelling enzymes to alter chromatin structure, many of which do so via nucleosome sliding. Nucleosome sliding is one of the possible mechanism for large scale tissue specific expression of genes. The transcription start site for genes expressed in a particular tissue, are nucleosome depleted while, the same set of genes in other tissue where they are not expressed, are nucleosome bound.[39]

DNA site exposure

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Nucleosomal DNA is in equilibrium between a wrapped and unwrapped state. DNA within the nucleosome remains fully wrapped for only 250 ms before it is unwrapped for 10-50 ms and then rapidly rewrapped, as measured using time-resolved FRET.[40] This implies that DNA does not need to be actively dissociated from the nucleosome but that there is a significant fraction of time during which it is fully accessible. Introducing a DNA-binding sequence within the nucleosome increases the accessibility of adjacent regions of DNA when bound.[41]

This propensity for DNA within the nucleosome to "breathe" has important functional consequences for all DNA-binding proteins that operate in a chromatin environment.[40] In particular, the dynamic breathing of nucleosomes plays an important role in restricting the advancement of RNA polymerase II during transcription elongation.[42]

Nucleosome free region

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Promoters of active genes have nucleosome free regions (NFR). This allows for promoter DNA accessibility to various proteins, such as transcription factors. Nucleosome free region typically spans for 200 nucleotides in S. cerevisiae[43] Well-positioned nucleosomes form boundaries of NFR. These nucleosomes are called +1-nucleosome and −1-nucleosome and are located at canonical distances downstream and upstream, respectively, from transcription start site.[44] +1-nucleosome and several downstream nucleosomes also tend to incorporate H2A.Z histone variant.[44]

Modulating nucleosome structure

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Eukaryotic genomes are ubiquitously associated into chromatin; however, cells must spatially and temporally regulate specific loci independently of bulk chromatin. In order to achieve the high level of control required to co-ordinate nuclear processes such as DNA replication, repair, and transcription, cells have developed a variety of means to locally and specifically modulate chromatin structure and function. This can involve covalent modification of histones, the incorporation of histone variants, and non-covalent remodelling by ATP-dependent remodeling enzymes.

Histone post-translational modifications

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Histone tails and their function in chromatin formation

Since they were discovered in the mid-1960s, histone modifications have been predicted to affect transcription.[45] The fact that most of the early post-translational modifications found were concentrated within the tail extensions that protrude from the nucleosome core lead to two main theories regarding the mechanism of histone modification. The first of the theories suggested that they may affect electrostatic interactions between the histone tails and DNA to "loosen" chromatin structure. Later it was proposed that combinations of these modifications may create binding epitopes with which to recruit other proteins.[46] Recently, given that more modifications have been found in the structured regions of histones, it has been put forward that these modifications may affect histone-DNA[47] and histone-histone[48] interactions within the nucleosome core. Modifications (such as acetylation or phosphorylation) that lower the charge of the globular histone core are predicted to "loosen" core-DNA association; the strength of the effect depends on location of the modification within the core.[49] Some modifications have been shown to be correlated with gene silencing; others seem to be correlated with gene activation. Common modifications include acetylation, methylation, or ubiquitination of lysine; methylation of arginine; and phosphorylation of serine. The information stored in this way is considered epigenetic, since it is not encoded in the DNA but is still inherited to daughter cells. The maintenance of a repressed or activated status of a gene is often necessary for cellular differentiation.[18]

Histone variants

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Although histones are remarkably conserved throughout evolution, several variant forms have been identified. This diversification of histone function is restricted to H2A and H3, with H2B and H4 being mostly invariant. H2A can be replaced by H2AZ (which leads to reduced nucleosome stability) or H2AX (which is associated with DNA repair and T cell differentiation), whereas the inactive X chromosomes in mammals are enriched in macroH2A. H3 can be replaced by H3.3 (which correlates with activate genes and regulatory elements) and in centromeres H3 is replaced by CENPA.[18]

ATP-dependent nucleosome remodeling

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A number of distinct reactions are associated with the term ATP-dependent chromatin remodeling. Remodeling enzymes have been shown to slide nucleosomes along DNA,[50] disrupt histone-DNA contacts to the extent of destabilizing the H2A/H2B dimer[51][52] and to generate negative superhelical torsion in DNA and chromatin.[53] Recently, the Swr1 remodeling enzyme has been shown to introduce the variant histone H2A.Z into nucleosomes.[54] At present, it is not clear if all of these represent distinct reactions or merely alternative outcomes of a common mechanism. What is shared between all, and indeed the hallmark of ATP-dependent chromatin remodeling, is that they all result in altered DNA accessibility.

Studies looking at gene activation in vivo[55] and, more astonishingly, remodeling in vitro[56] have revealed that chromatin remodeling events and transcription-factor binding are cyclical and periodic in nature. While the consequences of this for the reaction mechanism of chromatin remodeling are not known, the dynamic nature of the system may allow it to respond faster to external stimuli. A recent study indicates that nucleosome positions change significantly during mouse embryonic stem cell development, and these changes are related to binding of developmental transcription factors.[57]

Dynamic nucleosome remodelling across the Yeast genome

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Studies in 2007 have catalogued nucleosome positions in yeast and shown that nucleosomes are depleted in promoter regions and origins of replication.[58][59][60] About 80% of the yeast genome appears to be covered by nucleosomes[61] and the pattern of nucleosome positioning clearly relates to DNA regions that regulate transcription, regions that are transcribed and regions that initiate DNA replication.[62] Most recently, a new study examined dynamic changes in nucleosome repositioning during a global transcriptional reprogramming event to elucidate the effects on nucleosome displacement during genome-wide transcriptional changes in yeast (Saccharomyces cerevisiae).[63] The results suggested that nucleosomes that were localized to promoter regions are displaced in response to stress (like heat shock). In addition, the removal of nucleosomes usually corresponded to transcriptional activation and the replacement of nucleosomes usually corresponded to transcriptional repression, presumably because transcription factor binding sites became more or less accessible, respectively. In general, only one or two nucleosomes were repositioned at the promoter to effect these transcriptional changes. However, even in chromosomal regions that were not associated with transcriptional changes, nucleosome repositioning was observed, suggesting that the covering and uncovering of transcriptional DNA does not necessarily produce a transcriptional event. After transcription, the rDNA region has to protected from any damage, it suggested HMGB proteins play a major role in protecting the nucleosome free region.[64][65]

DNA Twist Defects

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DNA twist defects are when the addition of one or a few base pairs from one DNA segment are transferred to the next segment resulting in a change of the DNA twist. This will not only change the twist of the DNA but it will also change the length.[66] This twist defect eventually moves around the nucleosome through the transferring of the base pair, this means DNA twists can cause nucleosome sliding.[67] Nucleosome crystal structures have shown that superhelix location 2 and 5 on the nucleosome are commonly found to be where DNA twist defects occur as these are common remodeler binding sites.[68] There are a variety of chromatin remodelers but all share the existence of an ATPase motor which facilitates chromatin sliding on DNA through the binding and hydrolysis of ATP.[69] ATPase has an open and closed state. When the ATPase motor is changing from open and closed states, the DNA duplex changes geometry and exhibits base pair tilting.[68] The initiation of the twist defects via the ATPase motor causes tension to accumulate around the remodeler site. The tension is released when the sliding of DNA has been completed throughout the nucleosome via the spread of two twist defects (one on each strand) in opposite directions.[69]

Nucleosome assembly in vitro

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Diagram of nucleosome assembly

Nucleosomes can be assembled in vitro by either using purified native or recombinant histones.[70][71] One standard technique of loading the DNA around the histones involves the use of salt dialysis. A reaction consisting of the histone octamers and a naked DNA template can be incubated together at a salt concentration of 2 M. By steadily decreasing the salt concentration, the DNA will equilibrate to a position where it is wrapped around the histone octamers, forming nucleosomes. In appropriate conditions, this reconstitution process allows for the nucleosome positioning affinity of a given sequence to be mapped experimentally.[72]

Disulfide crosslinked nucleosome core particles

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A recent advance in the production of nucleosome core particles with enhanced stability involves site-specific disulfide crosslinks.[73] Two different crosslinks can be introduced into the nucleosome core particle. A first one crosslinks the two copies of H2A via an introduced cysteine (N38C) resulting in histone octamer which is stable against H2A/H2B dimer loss during nucleosome reconstitution. A second crosslink can be introduced between the H3 N-terminal histone tail and the nucleosome DNA ends via an incorporated convertible nucleotide.[74] The DNA-histone octamer crosslink stabilizes the nucleosome core particle against DNA dissociation at very low particle concentrations and at elevated salt concentrations.

Nucleosome assembly in vivo

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Steps in nucleosome assembly

Nucleosomes are the basic packing unit of genomic DNA built from histone proteins around which DNA is coiled. They serve as a scaffold for formation of higher order chromatin structure as well as for a layer of regulatory control of gene expression. Nucleosomes are quickly assembled onto newly synthesized DNA behind the replication fork.

H3 and H4

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Histones H3 and H4 from disassembled old nucleosomes are kept in the vicinity and randomly distributed on the newly synthesized DNA.[75] They are assembled by the chromatin assembly factor 1 (CAF-1) complex, which consists of three subunits (p150, p60, and p48).[76] Newly synthesized H3 and H4 are assembled by the replication coupling assembly factor (RCAF). RCAF contains the subunit Asf1, which binds to newly synthesized H3 and H4 proteins.[77] The old H3 and H4 proteins retain their chemical modifications which contributes to the passing down of the epigenetic signature. The newly synthesized H3 and H4 proteins are gradually acetylated at different lysine residues as part of the chromatin maturation process.[78] It is also thought that the old H3 and H4 proteins in the new nucleosomes recruit histone modifying enzymes that mark the new histones, contributing to epigenetic memory.

H2A and H2B

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In contrast to old H3 and H4, the old H2A and H2B histone proteins are released and degraded; therefore, newly assembled H2A and H2B proteins are incorporated into new nucleosomes.[79] H2A and H2B are assembled into dimers which are then loaded onto nucleosomes by the nucleosome assembly protein-1 (NAP-1) which also assists with nucleosome sliding.[80] The nucleosomes are also spaced by ATP-dependent nucleosome-remodeling complexes containing enzymes such as Isw1 Ino80, and Chd1, and subsequently assembled into higher order structure.[81][82]

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The crystal structure of the nucleosome core particle (PDB: 1EQZ​[28]) - different views showing details of histone folding and organization. Histones H2A, H2B, H3, H4 and DNA are coloured.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Nucleosome

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from Grokipedia
A nucleosome is the basic repeating subunit of in eukaryotic cells, consisting of approximately 147 base pairs of DNA wrapped in 1.65 left-handed superhelical turns around a central composed of two copies each of the core histones H2A, H2B, H3, and H4. This histone core has a diameter of about 11 nm, and the DNA-histone complex is connected by stretches of , typically 20–60 base pairs in length, with the often binding to stabilize the structure. In its extended form, the nucleosome array appears as "beads on a " under , representing the first level of DNA compaction within the nucleus. Nucleosomes play essential roles in and function beyond mere packaging, achieving an initial sevenfold linear compaction of DNA to fit the approximately 2 meters of human into a nucleus roughly 6 micrometers in . They regulate access to genetic information by modulating structure, influencing processes such as transcription, , repair, and recombination through dynamic assembly, disassembly, and repositioning. modifications, such as and , and variants of core histones further fine-tune nucleosome stability and function, enabling epigenetic control of . The nucleosome was first identified in 1974 by through biochemical reconstitution experiments that revealed its repeating nature in . The high-resolution of the nucleosome core particle, determined in 1997 at 2.8 Å resolution, confirmed the detailed architecture and histone-DNA interactions, providing a foundation for understanding dynamics. Since then, advances in have highlighted nucleosome plasticity, including breathing, sliding, and unwrapping, which are critical for cellular processes.

Structure

Core Particle Composition

The nucleosome serves as the fundamental subunit of chromatin, comprising approximately 147 base pairs of double-stranded DNA wrapped around a histone octamer formed by two copies each of the core histone proteins H2A, H2B, H3, and H4. This octamer represents the protein core of the nucleosome core particle, excluding the linker histone H1, which associates with the intervening DNA between nucleosomes. The core histones assemble into specific dimeric units: H2A-H2B heterodimers and H3-H4 heterodimers, with two H3-H4 dimers combining to form a central (H3-H4)2 tetramer that is flanked on either side by an H2A-H2B dimer, yielding the overall 2:2:2:2 stoichiometry of the octamer. This arrangement provides a stable scaffold for DNA packaging, with the stoichiometry ensuring one octamer per core particle. The atomic-level organization of the nucleosome core particle was first elucidated through X-ray crystallography at 2.8 Å resolution, revealing the detailed assembly of the histone octamer and its interactions with DNA. A subsequent refinement at 1.9 Å resolution provided further insights into solvent-mediated interactions and histone positioning within the core. Recent cryo-EM studies from 2023 to 2025 have confirmed and refined these structural features, validating that the nucleosome core particle has a diameter of about 11 nm and height of 5.7 nm, with the central histone octamer scaffold measuring approximately 6.5 nm in diameter and 5.7 nm in height.

Histone Organization and DNA Wrapping

The histone fold motif, a conserved structural element in core histones, consists of three alpha-helices connected by short loops within the globular domains, facilitating heterodimerization between H2A-H2B and H3-H4 pairs and subsequent assembly into the octamer. This motif's handshake-like configuration enables stable dimer formation, with the alpha-helices providing the primary surfaces for intermolecular contacts during octamer construction. The histone octamer forms a wedge-shaped scaffold, with a central (H3-H4)2 tetramer serving as the core around which two H2A-H2B dimers attach laterally. This architecture is stabilized by four key histone-histone interfaces: two between the H3-H4 dimers within the tetramer and two between the tetramer and each H2A-H2B dimer, primarily mediated by the histone fold domains and additional structured loops. The N-terminal tail domains of the histones protrude from this core, extending beyond the DNA wrapping region. Approximately 147 base pairs of DNA wrap around the octamer in 1.65-1.7 left-handed superhelical turns, forming the nucleosome core particle. This path positions the DNA backbone in close proximity to the histone surfaces, where it contacts roughly 89 residues across the octamer through predominantly electrostatic interactions between positively charged histone side chains and the negatively charged DNA phosphates. Stabilizing these contacts are arginine anchors, such as those from H3 and H4, which insert into the DNA minor groove at 10-14 sites along the superhelix, enhancing binding via shape readout and electrostatic attraction in narrowed groove regions. Complementary phosphate clamps, formed by histone loops (e.g., from H2B and H4), grip the DNA backbone at intervals, further securing the wrap and distributing bending strain. The DNA undergoes significant bending, averaging ~140° per turn around each pair, with local deformations driven by positive roll angles at minor groove-facing sites. The entry and exit points of the DNA are oriented approximately 80° apart relative to the octamer axis, reflecting the incomplete superhelical turn and facilitating formation without involvement here. Recent cryo-EM studies from 2024-2025 have uncovered subtle asymmetries in DNA-histone contacts, such as uneven distribution of insertions and interactions across the superhelix, which modulate local stability and influence nucleosome positioning preferences.

Histone Tails

Histone tails are flexible, unstructured extensions rich in positively charged and residues that protrude from the globular domains of the core in the nucleosome core particle (NCP). These tails, located at the N-termini of H2B, H3, and H4, and both N- and C-termini of H2A, lack a defined secondary structure and extend outward from the histone fold regions. For instance, the H3 N-terminal tail comprises approximately the first 40 residues, while the H4 N-terminal tail spans the initial 20 residues, enabling their mobility within the nucleosomal context. The positions of these tails allow them to emerge from specific points between the two gyres of DNA wrapped around the , facilitating interactions both within the same NCP (intra-nucleosomal) and with neighboring nucleosomes (inter-nucleosomal). In the NCP, tails such as the H3 N-tail can thread through the DNA superhelix, contacting multiple DNA segments, while others remain more solvent-exposed. These tails contribute to the of the NCP through weak, electrostatic interactions with nucleosomal DNA and adjacent histone surfaces, helping to compact and maintain the integrity of the core particle. A key example is the H4 N-terminal tail, which binds to the acidic patch on the H2A-H2B dimer within the octamer, reinforcing histone-DNA associations via charge complementarity. Similarly, the C-terminal tail of docks onto the surface of the H2B histone in the same nucleosome, stabilizing the dimer interface through hydrophobic and electrostatic contacts. The lengths and sequences of histone tails exhibit high evolutionary conservation, particularly in motifs enriched with basic residues that underpin their DNA-binding affinity and structural roles across eukaryotes. These conserved motifs, such as lysine-rich stretches in H3 and H4 tails, have persisted due to their essential contributions to nucleosome architecture. Recent cryo-electron (cryo-EM) studies have revealed that histone tails adopt a range of conformations within the NCP, ranging from DNA-bound states to more extended forms, which modulate nucleosome compactness while retaining partial order rather than complete disorder. These dynamic yet structured poses influence the overall stability of the core particle without disrupting the canonical histone-DNA wrapping. Histone tails serve as primary sites for post-translational modifications, such as and , which can subtly alter their interactions within the nucleosome.

Higher-Order Chromatin Folding

Nucleosomes are connected by stretches of , typically ranging from 20 to 80 base pairs in length, which plays a crucial role in determining the spacing between adjacent nucleosomes and influencing the overall architecture. This variability in linker length, observed across species, tissues, and even within individual genomes, modulates the flexibility and compaction potential of the nucleosome chain. Shorter linkers promote tighter packing, while longer ones allow for more extended conformations, affecting higher-order folding. The assembly of nucleosomes into higher-order structures has been proposed to involve the formation of the 30-nm fiber, a compacted form of the 11-nm nucleosomal array, although its existence as a regular structure remains controversial, with recent studies (as of 2025) suggesting more dynamic, irregular configurations such as melts or liquid-like domains. Two primary models describe this fiber: the model, featuring a one-start helical stacking of nucleosomes with winding around the , and the model, characterized by a two-start where nucleosomes alternate sides with straight, crossed segments. The binds to the entry and exit points of DNA on the nucleosome, stabilizing the fiber by neutralizing charges and promoting internucleosomal interactions, which is essential for achieving the 30-nm diameter. This H1-mediated compaction results in approximately a 6- to 7-fold reduction in length compared to the extended nucleosomal array, representing a key step in DNA packaging. Further compaction beyond the 30-nm fiber occurs during processes like mitosis, leading to metaphase chromosomes with an overall DNA packing ratio exceeding 10,000-fold relative to linear DNA, though the precise contribution from the 30-nm stage to this final state varies with additional looping and scaffolding. Recent studies have highlighted how nucleosome spacing fine-tunes higher-order chromatin folding. For instance, linker lengths around 25-30 bp alter fiber regularity and phase separation propensity, with shorter linkers (e.g., 25 bp) inducing irregular orientations that enhance inter-fiber interactions and liquid-liquid phase separation at lower salt concentrations, while 30 bp linkers favor compact, stable intra-fiber stacking. Additionally, advances in 2024-2025 reveal that heterogeneous nanoscopic packing domains, on the scale of tens to hundreds of nanometers, emerge through the interplay of nucleosome remodeling and loop extrusion mechanisms, creating conformationally diverse regions that deviate from uniform fiber models. Beyond the 30-nm fiber, organizes into looped domains mediated by factors like and , which extrude loops to form topologically associating domains (TADs) that constrain interactions and enhance compartmentalization. Phase-separated condensates represent another higher-order state, where multivalent interactions drive the formation of liquid-like droplets that concentrate and regulatory proteins, further compacting and segregating genomic regions. These structures collectively enable dynamic, hierarchical folding essential for organization.

Assembly

In Vitro Methods

One of the foundational techniques for in vitro nucleosome reconstitution is the salt dialysis method, which involves mixing purified histone octamers with DNA in a high-salt buffer (typically 2 M NaCl) to disrupt electrostatic interactions, followed by gradual dialysis to lower salt concentrations to physiological levels (around 150 mM), allowing the histones to spontaneously wrap DNA into nucleosome core particles. This approach yields well-folded nucleosomes with approximately 147 base pairs of DNA wrapped around the histone octamer, mimicking the core structure observed in vivo, and has been widely used since the 1970s for biophysical and structural studies. To facilitate sequential deposition of histone components, in vitro methods often incorporate histone chaperones such as nucleosome assembly protein 1 (NAP-1), which binds H2A-H2B dimers and promotes their transfer onto pre-assembled (H3-H4)₂ tetramers bound to DNA, enhancing assembly efficiency under low-salt conditions. NAP-1-mediated assembly avoids non-specific aggregation and allows for controlled incorporation of histone variants, producing positioned nucleosomes suitable for downstream assays. For enhanced stability in structural investigations, disulfide-crosslinked nucleosome cores are generated by engineering cysteine mutations at strategic positions in histone proteins, such as between H3 and H4 or H2A and H2B, followed by oxidation to form covalent bonds that lock the octamer in place after reconstitution via salt dialysis or chaperone-assisted methods. These stabilized particles resist disassembly during purification and enable high-resolution techniques like and cryo-electron microscopy (cryo-EM). Recent advancements (2023–2025) include the use of engineered cohesive-ended DNA fragments for assembling circular arrays, building on a 2021 method where short, complementary overhangs on nucleosomal DNA promote ligation into closed minicircles or arrays that maintain periodic nucleosome positioning without linker dependency. These constructs are particularly compatible with cryo-EM, as demonstrated in studies of variant nucleosomes and protein complexes, allowing visualization of dynamic conformations at near-atomic resolution while avoiding artifacts from linear arrays. Additionally, a 2024 method enables assembly of complete eukaryotic nucleosomes and (H3-H4)-only tetrasomes in by expressing core histones, mimicking stepwise eukaryotic deposition and facilitating production of non-canonical particles for structural studies. In vitro methods offer precise control over nucleosome positioning and composition for biophysical assays, such as single-molecule tracking of sliding dynamics, but they often exhibit non-physiological kinetics due to the absence of cellular regulatory factors. These techniques parallel assembly pathways in their stepwise deposition but lack the spatiotemporal regulation found in cells.

In Vivo Pathways

In vivo nucleosome assembly primarily occurs through replication-coupled pathways during , where parental histones are recycled and new histones are synthesized to maintain integrity behind the replication fork. This process ensures the duplication of nucleosomes in a manner coordinated with , preventing exposure of naked DNA and preserving epigenetic marks. The assembly begins with the deposition of an H3-H4 tetramer onto newly replicated DNA, facilitated by the chromatin assembly factor 1 (CAF-1) complex, which binds to (PCNA) at the replication fork. CAF-1 recognizes the replication-coupled context through this PCNA interaction, delivering the (H3-H4)2 tetramer to form a tetrasome intermediate that wraps approximately 70 base pairs of DNA. Subsequently, two H2A-H2B dimers are added sequentially to complete the octameric nucleosome core particle, with chaperones like Nap1 or sNASP aiding in dimer placement. Beyond replication, nucleosomes assemble through replication-independent pathways during processes such as transcription elongation or , where different histone chaperones mediate histone deposition to restore structure. For instance, during transcription, the FACT complex recycles and reassembles nucleosomes to facilitate passage, while in double-strand break repair, the HIRA chaperone deposits H3.3-containing nucleosomes to fill gaps. Recent structural studies from 2024 have elucidated the molecular details of CAF-1's histone binding, revealing how its disordered regions and folded modules interact with H3-H4 dimers to ensure precise deposition and epigenetic inheritance during replication. These insights highlight CAF-1's conformational dynamics upon histone binding, which modulate its affinity for PCNA and promote faithful transmission of parental histone modifications to daughter strands.

Chaperone Involvement

Histone chaperones play a crucial role in nucleosome assembly by binding soluble , preventing their nonspecific aggregation, and guiding their sequential deposition onto without becoming integral components of the final nucleosome structure. These proteins ensure the fidelity of formation during processes such as and repair, maintaining epigenetic information and integrity. Chaperones are categorized based on their histone specificity: those dedicated to H3-H4, such as ASF1 and CAF-1, and those for H2A-H2B, including NAP-1 and FACT. ASF1 binds the H3-H4 heterodimer through interaction with the alpha N helix of , stabilizing the dimer and facilitating its delivery to form the tetramer core. This binding prevents histone aggregation and promotes ordered deposition, while CAF-1 further integrates the H3-H4 tetramer into nascent . For H2A-H2B, NAP-1 sequesters dimers to eliminate competing non-nucleosomal histone-DNA interactions, enabling efficient dimer addition to the H3-H4 core. FACT, a heterodimer of Spt16 and Pob3 (or SSRP1 in humans), similarly chaperones H2A-H2B and supports both assembly and disassembly by modulating interactions during dynamic transactions. Specificity distinguishes chaperone functions: CAF-1 is specialized for replication-coupled assembly, associating with the PCNA sliding clamp to deposit histones onto newly synthesized DNA strands. In contrast, NAP-1 operates more generally, facilitating nucleosome assembly in diverse contexts beyond replication, such as transcription-associated events. ASF1 and FACT exhibit broader roles but cooperate with these partners; for instance, ASF1 hands off H3-H4 to CAF-1 during replication. Recent structural advances, including cryo-EM studies from 2024-2025, have revealed transient chaperone-nucleosome intermediates, such as FACT-bound hexasome-like states during assembly, highlighting dynamic, stepwise interactions that preserve nucleosome integrity. These chaperones are evolutionarily conserved across eukaryotes, from yeast to humans, underscoring their essential role in genome stability by preventing chromatin defects that could lead to replication stress or mutations. Depletion of ASF1, CAF-1, NAP-1, or FACT components results in hypersensitivity to DNA damage and chromosomal instability, emphasizing their non-redundant contributions to faithful nucleosome biogenesis.

Dynamics

Nucleosome Sliding and Positioning

Nucleosome sliding refers to the repositioning of the histone octamer along the DNA, enabling dynamic adjustments in chromatin structure that influence DNA accessibility. This process encompasses both passive and active modes, where the nucleosome translocates without dissociation from the DNA. Positioning, in turn, describes the stable placement of nucleosomes at specific genomic loci, guided by intrinsic DNA properties and extrinsic factors. These movements are crucial for maintaining chromatin organization, though detailed modulation by remodeling complexes is addressed elsewhere. The sliding mechanism involves coupled rotational and translational shifts of the nucleosome along the DNA axis. Translational movement occurs in discrete steps of approximately 5-10 s, during which the DNA twists around the core in a screw-like fashion, requiring a of about 36° per base pair shift to maintain histone-DNA contacts. Passive sliding is driven by , allowing spontaneous, diffusion-like repositioning without external energy input, often observed under physiological conditions like elevated or . In contrast, active sliding is powered by , primarily facilitated by complexes that propagate DNA twists or bulge propagation to shift the nucleosome directionally. Nucleosome positioning is influenced by DNA sequence motifs that act as signals for exclusion or stabilization. Poly(dA:dT) tracts, rich in adenine-thymine base pairs, strongly disfavor nucleosome formation due to their rigid, straight helical structure, thereby creating nucleosome-free regions particularly at gene promoters. In barrier models, these tracts serve as boundaries that statistically position adjacent nucleosomes by limiting their mobility and promoting ordered arrays. Experimental evidence for sliding dynamics has been provided by fluorescence resonance energy transfer (FRET) and single-molecule techniques, which track nucleosome movements in real time. These studies reveal sliding rates of approximately 1-10 base pairs per second for active processes, with passive rates being significantly slower due to reliance on thermal energy. Recent advances in cryo-electron microscopy (cryo-EM) from 2023 to 2025 have captured sliding intermediates, elucidating transient histone-DNA distortions that facilitate translocation. For instance, structures of remodeler-nucleosome complexes show twisted DNA configurations and partial unwrapping during the sliding cycle, highlighting the molecular basis of these distortions.

DNA Site Exposure and Breathing

The breathing model describes the spontaneous, reversible partial unwrapping and rewrapping of DNA segments from the ends of the nucleosome core particle, typically involving 10-20 base pairs (bp) at each entry and exit site. This dynamic equilibrium favors the wrapped state, with an for unwrapping of approximately 0.2-0.6 at the nucleosome ends under physiological conditions. Such fluctuations arise from and the elastic properties of DNA-histone interactions, without requiring external factors like ATP-dependent remodelers. These unwrapping events result in transient site exposure, rendering approximately 20-30 bp of DNA accessible at the entry and exit regions, which can facilitate binding of transcription factors to otherwise occluded sequences. For instance, pioneer transcription factors exploit this exposure to initiate access to promoter regions, as the partially unwrapped DNA segments provide a kinetic window for protein-DNA interactions before rewrapping occurs. The kinetics of breathing occur on millisecond timescales, with unwrapped state lifetimes ranging from 10-50 , enabling rapid equilibrium between wrapped and unwrapped conformations. significantly influences these dynamics; higher salt concentrations (50-100 mM NaCl) promote unwrapping by screening electrostatic interactions between DNA and histones, increasing the probability of the open state by up to twofold compared to low-salt conditions. Single-molecule (smFRET) experiments have provided direct evidence for these partial unwrapping probabilities, revealing that 10-20% of nucleosomes exhibit intermediate unwrapped states at the ends, with probabilities decreasing sharply toward the dyad axis (e.g., ~10% at 27 inward). These measurements, combined with gel-based assays, confirm the site-specific nature of breathing and its role in modulating DNA accessibility. Recent advances in 2025 have integrated cryo-electron microscopy (cryo-EM) data with computational modeling to elucidate the structural basis of unwrapping dynamics, capturing heterogeneous conformations and histone rearrangements during partial DNA dissociation.00060-7) This integrative approach highlights how local histone flexibility contributes to the energy landscape of breathing, offering atomic-level insights into the process.

Nucleosome-Free Regions

Nucleosome-free regions (NFRs), also termed nucleosome-depleted regions (NDRs), represent stable segments of eukaryotic DNA that lack nucleosomes, particularly at gene promoters where they span approximately 140–200 base pairs.00257-8)00769-8) These regions are typically flanked by precisely positioned nucleosomes, including the -1 nucleosome upstream and the +1 nucleosome downstream, which together define a characteristic chromatin architecture that facilitates regulatory access to DNA.00257-8) The formation of NFRs arises through intrinsic and extrinsic mechanisms. Intrinsically, certain DNA sequences act as barriers to nucleosome assembly; for instance, poly(dA:dT) tracts, which are rigid and AT-rich, inherently resist nucleosome wrapping due to their biophysical properties, thereby promoting nucleosome exclusion in promoter regions. Extrinsically, factors such as bound proteins can evict or prevent nucleosome occupancy, maintaining the open configuration of these regions. Temporary nucleosome breathing at the edges of NFRs may contribute to their boundaries but does not account for their persistence. NFRs are detected genome-wide using micrococcal nuclease sequencing (MNase-seq), which reveals regions of hypersensitivity to nuclease digestion due to low nucleosome occupancy and high chromatin accessibility. In these assays, NFRs appear as valleys of protection in nucleosome occupancy profiles, contrasting with the periodic peaks from wrapped DNA elsewhere. Biologically, NFRs are crucial for transcription initiation, providing an accessible platform for transcription factors and the pre-initiation complex to bind upstream of the transcription start site, with the downstream +1 nucleosome helping to define the precise positioning for polymerase entry. Recent studies have linked NFRs to broader chromatin organization, showing that their presence contributes to conformationally defined heterogeneous packing domains through interactions with transcription and nucleosome dynamics.

Modulation and Remodeling

Histone Post-Translational Modifications

Histone post-translational modifications (PTMs) occur primarily on the N-terminal tails of core histones H2A, H2B, H3, and H4, as well as on their globular domains, altering the nucleosome's interaction with DNA and other proteins. These covalent changes include acetylation, methylation, phosphorylation, and ubiquitination, each mediated by specific enzymes that add or remove the modifications to regulate chromatin structure. Acetylation involves the addition of acetyl groups to lysine residues by histone acetyltransferases (HATs), such as p300/CBP, neutralizing the positive charge and reducing the electrostatic affinity of histone tails for negatively charged DNA, which loosens nucleosome packing and promotes a more open chromatin conformation. Deacetylation is catalyzed by histone deacetylases (HDACs), restoring the charge and tightening interactions. Key sites include H3K9 and H4K16, where acetylation at H4K16 disrupts internucleosomal contacts, facilitating chromatin fiber decompaction. Methylation adds methyl groups to lysine or arginine residues via histone methyltransferases (HMTs), such as SET1 for H3K4 or EZH2 for H3K27, and can be mono-, di-, or tri-methylated, influencing nucleosome stability without altering charge. For instance, H3K4me3 correlates with euchromatin openness by recruiting reader proteins that stabilize accessible structures, while H3K27me3 compacts chromatin through enhanced tail interactions with adjacent nucleosomes. Demethylation is performed by enzymes like LSD1 or JMJD family members. Phosphorylation adds phosphate groups to serine, threonine, or tyrosine residues by kinases, introducing negative charge that can repel DNA or recruit effectors, altering nucleosome positioning. Ubiquitination conjugates ubiquitin to lysines, often monoubiquitination on H2B K120, which sterically hinders tail folding and modulates nucleosome array compaction.00115-6) The "histone code" hypothesis posits that combinations of these PTMs form a combinatorial interpreted by chromatin-associated factors, dictating nucleosome and accessibility beyond individual marks. Crosstalk between modifications ensures specificity; for example, H3K4 sterically or allosterically blocks H3K9 by competing for the same or reader domains, preventing repressive compaction at active loci.00115-6) Recent studies (2023–2025) highlight how PTMs on non-canonical nucleosomes, such as those with histone variants, expand structural diversity by modulating enzyme access and tail dynamics in specialized chromatin contexts. Histone variants can subtly alter modification sites, influencing PTM patterns as detailed in subsequent sections.

Histone Variants and Non-Canonical Forms

Histone variants are sequence-divergent isoforms of the core that replace their counterparts within nucleosomes, thereby conferring specialized structural and functional properties to . histone H3.1 is primarily incorporated in a replication-coupled manner during to maintain integrity post-DNA synthesis, whereas the variant H3.3 is deposited in a replication-independent fashion, predominantly at transcriptionally active regions to facilitate . Similarly, H2A.Z marks nucleosomes in dynamic environments, such as promoter and enhancer regions prone to frequent remodeling, while CENP-A specifically localizes to centromeric nucleosomes to establish epigenetic identity. These variants differ from histones by only a few but elicit profound effects on nucleosome stability and accessibility. Structurally, H2A.Z introduces alterations in the that reduce DNA wrapping affinity, leading to more unstable nucleosomes with increased propensity for partial unwrapping and histone exchange compared to canonical H2A-containing nucleosomes. For instance, the extended acidic patch in H2A.Z enhances interactions with remodeling factors while destabilizing the DNA-histone interface, promoting open chromatin conformations. In contrast, CENP-A replaces H3 in centromeric nucleosomes, featuring a unique targeting domain that recruits proteins like CENP-C and CENP-N, thereby directing the assembly of the mitotic machinery essential for chromosome segregation. Post-translational modifications on these variants can further enhance their specificity, such as on H3.3 to promote binding. Non-canonical nucleosomes arise from incomplete histone octamers or variant incorporations, including hexasomes (lacking one H2A-H2B dimer), tetrasomes (lacking both H2A-H2B dimers), and variant-specific octamers, all of which exhibit altered stability to facilitate transient DNA exposure. Hexasomes, for example, display asymmetric DNA wrapping and reduced thermal stability, enabling ATP-dependent remodelers like INO80 to reposition them during transcription initiation. These forms represent intermediate states in nucleosome disassembly or assembly, with variant-octamers such as those containing H2A.Z showing heightened lability that correlates with faster histone turnover rates. The incorporation of histone variants relies on dedicated chaperones that ensure precise deposition. For H3.3, the chaperone DAXX, often in complex with , specifically recognizes and delivers H3.3-H4 dimers to target loci like telomeres and pericentromeric , bypassing replication timing. This chaperone-mediated pathway contrasts with replication-coupled assembly of canonical , allowing H3.3 to maintain active or repressed states independently of . Recent structural studies have illuminated the diversity of non-canonical nucleosomes in overcoming transcription barriers, revealing how hexasomes and variant-octamers enable progression by transiently exposing DNA while preserving overall integrity. Cryo-EM analyses in 2025 have further detailed subnucleosome intermediates involving CENP-A, underscoring their role in maturation beyond canonical centromeric functions.

ATP-Dependent Remodeling Complexes

ATP-dependent chromatin remodeling complexes are multi-subunit enzymes that utilize the energy from ATP hydrolysis to reposition, evict, or exchange nucleosomes, thereby regulating DNA accessibility for processes such as transcription and replication. These complexes are classified into four major families based on the structure of their central ATPase subunit: SWI/SNF, ISWI, CHD, and INO80. The SWI/SNF family, including yeast Swi/Snf and mammalian BAF and PBAF complexes, primarily facilitates nucleosome sliding and eviction to promote gene activation. In contrast, ISWI complexes, such as ACF and CHRAC, specialize in nucleosome spacing and assembly, creating regularly spaced arrays that maintain chromatin organization. CHD family remodelers, like Chd1 and NuRD, focus on nucleosome repositioning and are often involved in transcriptional repression or elongation. The INO80 family, encompassing INO80 and SWR1 complexes, excels in histone variant exchange and nucleosome eviction, particularly at sites of DNA damage or promoters. The core mechanism of these complexes involves the ATPase domain, which belongs to the SNF2 superfamily, binding to nucleosomal DNA and using ATP hydrolysis to translocate the DNA relative to the histone octamer, thereby disrupting histone-DNA contacts. This translocation is thought to proceed via the "bulge propagation" model, where the ATPase induces a bulge or loop in the DNA at the entry site to the nucleosome; propagation of this bulge along the nucleosome surface shifts the DNA by several base pairs, effectively sliding or restructuring the nucleosome. Each cycle of ATP binding, hydrolysis, and release typically advances the DNA by approximately 1-2 base pairs, with the process being directional (often 3' to 5' along one DNA strand) and coupled to conformational changes in the ATPase's RecA-like lobes. These complexes act on both canonical nucleosomes containing histone H3.1/H3.2 and variant nucleosomes, such as those with H2A.Z or H3.3, allowing targeted remodeling at specific genomic loci. A prominent example is the SWR1 complex, which catalyzes the ATP-dependent exchange of canonical H2A-H2B dimers for H2A.Z-H2B dimers in nucleosomes, preferentially at promoter regions to facilitate transcription ; this process involves sequential steps for dimer eviction and insertion without net nucleosome disassembly. Recent studies have revealed that ATP-dependent remodelers cooperate with loop extrusion factors, such as , to define heterogeneous nanoscopic packing domains (~100-500 nm), where remodeling activity modulates local nucleosome density and influences patterns in three-dimensional nuclear space.

Genome-Wide Dynamics and DNA Defects

Genome-wide mapping techniques such as and have revealed periodic nucleosome positioning patterns across eukaryotic genomes, highlighting the organized arrangement of nucleosomes in . involves partial digestion of with micrococcal , which preferentially cleaves , allowing high-resolution profiling of nucleosome occupancy and positioning from sequencing reads of protected DNA fragments. Similarly, assesses accessibility by transposase-mediated insertion of tags into open regions, indirectly delineating nucleosome positions through protection of wrapped DNA and exposure of linkers or depleted areas. These methods demonstrate that nucleosomes often exhibit phased arrays with repeat lengths of approximately 160-200 base pairs, influenced by underlying DNA sequence preferences and regulatory elements. In yeast models, such as , genome-wide analyses uncover dynamic nucleosome remodeling characterized by high turnover rates, particularly at promoters where nucleosomes are rapidly repositioned to facilitate gene activation. Studies using time-resolved nucleosome occupancy profiling show that promoter regions display fluctuating nucleosome configurations, with remodelers driving and reassembly in response to environmental cues, leading to transient exposure of binding sites. For instance, at the PHO5 promoter, nucleosome dynamics involve sequential remodeling events that alter array stability, contributing to a broader of high-mobility nucleosomes genome-wide. This turnover is more pronounced in yeast compared to higher eukaryotes, reflecting a compact organization adapted for rapid transcriptional responses. Nucleosome spacing varies across species, with yeast exhibiting tighter packing than mammals, which influences overall chromatin compaction and accessibility. In S. cerevisiae, the average nucleosome repeat length is about 165 base pairs, comprising 147 base pairs of wrapped DNA and a 18-base-pair linker, enabling dense arrays suited to the smaller genome. In contrast, mammalian cells show longer linkers averaging 40-60 base pairs, resulting in repeat lengths of 180-200 base pairs and more variable positioning that accommodates diverse regulatory demands. These differences arise from evolutionary adaptations in linker histone incorporation and remodeler activity, with yeast relying less on H1 variants for spacing control. DNA twist defects, arising from mismatches in helical phasing between nucleosomal DNA and linker regions, introduce strain that affects nucleosome positioning and mobility. These defects occur when the ~10.5 base pairs per turn of free DNA adjusts to the histone octamer's geometry, which over-twists DNA at certain superhelix locations, propagating as localized kinks or bulges. Such strain facilitates nucleosome sliding by allowing twist propagation around the histone core, influencing genome-wide positioning signals and enabling responses to remodeling forces without full disassembly. In non-canonical nucleosome forms, twist defects are exacerbated, leading to altered conformations that impact chromatin higher-order structures. Recent studies in 2025 have elucidated the interplay between nucleosome remodeling, transcription, and loop in shaping conformation at the genome scale. Integrating computational modeling with experimental data, these works show that coordinated remodeling and extrusion activities generate heterogeneous nanoscopic packing domains, where nucleosome arrays dynamically adjust to transcriptional waves. This emergent system highlights how twist defects in non-canonical nucleosomes contribute to conformational flexibility, particularly in regulatory hotspots, advancing understanding of large-scale dynamics.

Biological Functions

Role in Gene Transcription

Nucleosomes act as barriers to (RNAPII) transcription by wrapping DNA, which impedes the enzyme's access to promoter regions and progression along the gene body. This barrier function is particularly pronounced at transcription initiation, where nucleosomes must be repositioned or evicted to allow RNAPII assembly into pre-initiation complexes. Studies using single-molecule techniques have shown that RNAPII pauses at nucleosomal boundaries, requiring energy-dependent mechanisms to proceed, as demonstrated in assays with reconstituted templates. In promoter architecture, nucleosome-free regions (NFRs) upstream of the transcription start site (TSS) facilitate RNAPII , while the positioned +1 nucleosome downstream of the TSS precisely defines the start site by occluding alternative initiation points. Genome-wide mapping in reveals that NFRs are enriched with AT-rich sequences that resist nucleosome formation, positioning the +1 nucleosome to modulate promoter clearance and ensure accurate TSS selection. This organization is conserved across eukaryotes, with the +1 nucleosome often exhibiting rotational phasing that influences RNAPII pausing and productive elongation. During transcription elongation, the FACT complex plays a central role by temporarily disassembling nucleosomes ahead of RNAPII and reassembling them behind, preventing stable barriers to progression. FACT binds to H2A-H2B dimers, facilitating their eviction during elongation and redeposition post-passage, as evidenced by biochemical assays showing enhanced RNAPII processivity on nucleosomal templates in the presence of FACT. This dynamic nucleosome management is essential for efficient , with FACT associating with elongating RNAPII . Epigenetic modifications on nucleosomal histones further regulate transcription by altering accessibility. of at 4 (H3K4ac) is enriched at active promoters, promoting an open state that facilitates RNAPII and elongation by recruiting bromodomain-containing factors. In contrast, trimethylation of H3 at 27 (H3K27me3), mediated by Polycomb repressive complex 2, compacts nucleosomes and blocks RNAPII access, enforcing transcriptional repression at developmental genes. These marks often coexist in bivalent domains, balancing poised states for rapid activation or silencing. Recent studies highlight the role of non-canonical nucleosomes, such as tetrasomes and hexasomes, in overcoming transcription barriers. These subnucleosomal particles, formed during RNAPII passage, exhibit structural flexibility that allows polymerase traversal without full disassembly, as revealed by cryo-EM structures of RNAPII on H3-H4 tetrasomes. In , heterogeneous non-canonical forms predominate , enabling dynamic responses to transcriptional demands and integrating with remodeling factors for barrier resolution.

Involvement in DNA Replication and Repair

During DNA replication, nucleosomes positioned ahead of the advancing replication fork must be disassembled to permit the replisome to access and unwind the underlying DNA. ATP-dependent chromatin remodeling complexes, including the ACF (ATP-utilizing chromatin assembly and remodeling factor) and RSF (remodeling and spacing factor), play key roles in this disassembly process by mobilizing or evicting histones, thereby facilitating fork progression. Behind the fork, newly synthesized DNA is rapidly repackaged into nucleosomes through replication-coupled assembly mediated primarily by the chromatin assembly factor 1 (CAF-1) complex, which deposits histone H3-H4 dimers in a PCNA-dependent manner to restore chromatin structure. This coordinated disassembly and reassembly ensures efficient duplication of the genome while maintaining chromatin integrity. A critical aspect of replication is the preservation of epigenetic information through the recycling of parental histones. Modified parental H3-H4 tetramers are randomly but symmetrically segregated to the two daughter DNA strands, allowing post-translational marks to be transferred and inherited, which supports stable epigenetic memory across cell divisions. This process involves histone chaperones that facilitate the redeposition of parental histones alongside newly synthesized ones, preventing dilution of epigenetic states during chromatin duplication. In DNA repair, nucleosomes at damage sites undergo dynamic remodeling, including histone exchange, to expose lesions for repair machinery access. For instance, at double-strand breaks (DSBs), the INO80 chromatin remodeling complex promotes histone exchange by evicting H2A.Z-containing nucleosomes and replacing them with canonical H2A, which facilitates subsequent repair steps. Additionally, the histone variant H2AX within nucleosomes is rapidly phosphorylated at serine 139 (forming γH2AX) by kinases such as ATM and DNA-PK, generating signaling platforms that recruit repair factors like 53BP1 and BRCA1 to amplify the DNA damage response. Nucleosome eviction is particularly important at DSBs to enable repair pathway choice and execution. Eviction of nucleosomes near the break exposes DNA ends, promoting access for (NHEJ) in or (HR) in S/, with remodeling complexes like INO80 and aiding in this transient opening. Recent structural studies have elucidated the mechanism of CAF-1 in replication-coupled assembly, revealing how its subunits bind acetylated H3-H4 and interact with PCNA to deposit nucleosomes efficiently behind the fork, with implications for both replication fidelity and repair contexts.

Contribution to Chromosome Condensation and Stability

Nucleosomes play a central role in condensation by facilitating the higher-order folding of into loops and scaffolds, particularly during mitosis, which compacts the to enable efficient segregation. This process involves the , which binds to nucleosome and promotes the compaction of nucleosome arrays into 30-nm fibers and further structures, stabilizing interactions that reduce volume by orders of magnitude. Similarly, (HP1) contributes by binding to tails methylated at lysine 9 (H3K9me), bridging adjacent nucleosomes to form condensed domains essential for mitotic architecture. These mechanisms ensure that chromosomes achieve the necessary density for spindle attachment and movement without entanglement. Beyond condensation, nucleosomes enhance chromosome stability by preventing DNA tangling and shielding the genome from nuclease degradation. The wrapping of DNA around histone octamers constrains supercoiling and reduces the risk of knots or catenanes during chromosome segregation, maintaining structural integrity across cell divisions. Additionally, nucleosome assembly inherently protects DNA from endonucleases, as the core particle sequesters approximately 146 base pairs of DNA, rendering it inaccessible to degradative enzymes and thereby safeguarding genomic material . At centromeres, specialized nucleosomes containing the histone H3 variant CENP-A serve as the epigenetic foundation for assembly, enabling precise attachment during . CENP-A nucleosomes recruit constitutive centromere-associated network (CCAN) proteins, which in turn stabilize the inner and facilitate bioriented alignment, a process critical for error-free segregation. This variant replaces canonical H3 in centromeric , altering nucleosome stability to support the dynamic yet robust interactions required for spindle function. In , nucleosomes incorporating the histone variant H3.3 contribute to end protection by maintaining heterochromatic structures that prevent DNA damage and fusions. The deposition of H3.3 at telomeric repeats, mediated by chaperones like ATRX-DAXX, promotes trimethylation of H3 lysine 9 (), which recruits protective factors and suppresses recombination, thereby preserving telomere length and chromosome stability over multiple divisions. Recent studies have highlighted how nucleosome spacing influences higher-order assembly and stability through mechanisms. Variations in length, tunable at single base-pair resolution, modulate internucleosomal interactions to either promote or inhibit liquid-like , fine-tuning compaction and resistance to mechanical stress in mitotic chromosomes. Furthermore, phase-separated domains involving nucleosomes and associated proteins enhance overall stability by compartmentalizing , reducing diffusion, and protecting against genotoxic insults.

References

  1. https://www.nature.com/scitable/topicpage/dna-packaging-nucleosomes-and-chromatin-310
  2. https://pmc.ncbi.nlm.nih.gov/articles/PMC4598263/
  3. https://www.nature.com/scitable/definition/nucleosome-nucleosomes-30/
  4. https://pmc.ncbi.nlm.nih.gov/articles/PMC3125042/
  5. https://www.genome.gov/genetics-glossary/Nucleosome
  6. https://pmc.ncbi.nlm.nih.gov/articles/PMC4378457/
  7. https://pmc.ncbi.nlm.nih.gov/articles/PMC5831138/
  8. https://pmc.ncbi.nlm.nih.gov/articles/PMC10096840/
  9. https://pubmed.ncbi.nlm.nih.gov/10458604/
  10. https://www.nature.com/articles/38444
  11. https://pmc.ncbi.nlm.nih.gov/articles/PMC7386248/
  12. https://www.nature.com/articles/srep03510
  13. https://www.nature.com/articles/s41467-023-42072-1
  14. https://pmc.ncbi.nlm.nih.gov/articles/PMC2946858/
  15. https://www.gs.washington.edu/academics/courses/braun/55104/readings/luger.pdf
  16. https://www.pnas.org/doi/10.1073/pnas.95.24.14173
  17. https://febs.onlinelibrary.wiley.com/doi/10.1016/j.fob.2013.08.007
  18. https://pmc.ncbi.nlm.nih.gov/articles/PMC4269774/
  19. https://www.sciencedirect.com/science/article/pii/S2589004222002073
  20. https://www.embopress.org/doi/10.1093/emboj/20.18.5207
  21. https://pmc.ncbi.nlm.nih.gov/articles/PMC3375473/
  22. https://www.pnas.org/doi/10.1073/pnas.032664299
  23. https://www.biorxiv.org/content/10.1101/2025.01.16.633411.full
  24. https://www.pnas.org/doi/10.1073/pnas.0802336105
  25. https://www.sciencedirect.com/science/article/pii/S0006349514010005
  26. https://pmc.ncbi.nlm.nih.gov/articles/PMC2267236/
  27. https://www.nature.com/articles/s41422-024-01009-z
  28. https://www.sciencedirect.com/science/article/abs/pii/S0968000410001696
  29. https://www.nature.com/articles/s41467-025-61482-x
  30. https://www.science.org/doi/10.1126/sciadv.adq6652
  31. https://www.pnas.org/doi/10.1073/pnas.1518552112
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC9956262/
  33. https://pmc.ncbi.nlm.nih.gov/articles/PMC8054172/
  34. https://www.sciencedirect.com/science/article/abs/pii/B9780123919380000094
  35. https://pmc.ncbi.nlm.nih.gov/articles/PMC2880918/
  36. https://www.sciencedirect.com/science/article/pii/S0021925820558249
  37. https://pmc.ncbi.nlm.nih.gov/articles/PMC5757783/
  38. https://pubmed.ncbi.nlm.nih.gov/29113904/
  39. https://academic.oup.com/nar/article/49/9/e52/6137298
  40. https://www.nature.com/articles/s41586-024-07196-4
  41. https://www.nature.com/articles/s42003-024-07211-4
  42. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0103752
  43. https://www.cell.com/cell/pdf/S0092-8674%2816%2931330-7.pdf
  44. https://cellandbioscience.biomedcentral.com/articles/10.1186/s13578-020-00398-z
  45. https://www.sciencedirect.com/science/article/pii/S0092867400806613
  46. https://academic.oup.com/nar/article/51/8/3770/7081434
  47. https://pmc.ncbi.nlm.nih.gov/articles/PMC3885639/
  48. https://pmc.ncbi.nlm.nih.gov/articles/PMC4004355/
  49. https://www.sciencedirect.com/science/article/abs/pii/S0968000423002062
  50. https://elifesciences.org/articles/15129
  51. https://pmc.ncbi.nlm.nih.gov/articles/PMC11186048/
  52. https://elifesciences.org/articles/91461
  53. https://www.cell.com/fulltext/S0092-8674%2810%2900005-X
  54. https://www.cell.com/cell/fulltext/S0092-8674%2806%2901273-6
  55. https://www.nature.com/articles/srep26702
  56. https://www.cell.com/molecular-cell/fulltext/S1097-2765%2810%2900156-5
  57. https://www.cell.com/molecular-cell/fulltext/S1097-2765%2818%2930465-9
  58. https://www.cell.com/fulltext/S0092-8674%2800%2980661-3
  59. https://www.pnas.org/doi/10.1073/pnas.0508002103
  60. https://www.cell.com/cell/pdf/S0092-8674%2810%2900005-X.pdf
  61. https://www.cell.com/molecular-cell/fulltext/S1097-2765%2825%2900407-1
  62. https://pmc.ncbi.nlm.nih.gov/articles/PMC2981792/
  63. https://www.pnas.org/doi/10.1073/pnas.1635158100
  64. https://www.mdpi.com/1422-0067/24/2/1051
  65. https://www.embopress.org/doi/10.1093/emboj/cdf486
  66. https://journals.plos.org/ploscompbiol/article?id=10.1371/journal.pcbi.1005880
  67. https://genomebiology.biomedcentral.com/articles/10.1186/gb-2010-11-5-r51
  68. https://academic.oup.com/nar/article/46/6/2788/4913576
  69. https://pmc.ncbi.nlm.nih.gov/articles/PMC126283/
  70. https://www.nature.com/articles/s41467-021-22636-9
  71. https://pmc.ncbi.nlm.nih.gov/articles/PMC2947954/
  72. https://pmc.ncbi.nlm.nih.gov/articles/PMC2830277/
  73. https://www.sciencedirect.com/science/article/abs/pii/S0022283606007960
  74. https://pmc.ncbi.nlm.nih.gov/articles/PMC2493396/
  75. https://www.sciencedirect.com/science/article/pii/S1097276506007313
  76. https://www.embopress.org/doi/10.1038/emboj.2011.141
  77. https://www.cell.com/molecular-cell/fulltext/S1097-2765%2825%2900362-4
  78. https://www.cell.com/biophysj/fulltext/S0006-3495(09)00849-2
  79. https://academic.oup.com/nar/article/51/3/1139/6997964
  80. https://www.pnas.org/doi/10.1073/pnas.1815424116
  81. http://mobydick.ucsf.edu/~haoli/genome_research_Wu_2010.pdf
  82. https://genomebiology.biomedcentral.com/articles/10.1186/s13059-019-1815-z
  83. https://www.nature.com/articles/ncomms11485
  84. https://www.sciencedirect.com/science/article/pii/S0092867409002578
  85. https://www.sciencedirect.com/science/article/pii/S0092867409004516
  86. https://pmc.ncbi.nlm.nih.gov/articles/PMC10200003/
  87. https://www.pnas.org/doi/10.1073/pnas.2301063120
  88. https://pmc.ncbi.nlm.nih.gov/articles/PMC10602921/
  89. https://www.nature.com/articles/cr201122
  90. https://www.nature.com/articles/emm201711
  91. https://www.sciencedirect.com/science/article/pii/S235230422200099X
  92. https://www.pnas.org/doi/10.1073/pnas.1017374108
  93. https://www.sciencedirect.com/science/article/abs/pii/S0959440X23001239
  94. https://pmc.ncbi.nlm.nih.gov/articles/PMC10903488/
  95. https://www.nature.com/articles/cr201132
  96. https://portlandpress.com/essaysbiochem/article/63/1/45/173/Nucleosome-remodelling-structural-insights-into
  97. https://febs.onlinelibrary.wiley.com/doi/10.1111/j.1742-4658.2011.08281.x
  98. https://pmc.ncbi.nlm.nih.gov/articles/PMC8127953/
  99. https://www.nature.com/articles/s41467-022-34861-x
  100. https://pmc.ncbi.nlm.nih.gov/articles/PMC10895462/
  101. https://www.frontiersin.org/journals/genetics/articles/10.3389/fgene.2020.618478/full
  102. https://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0060065
  103. https://pmc.ncbi.nlm.nih.gov/articles/PMC8004102/
  104. https://www.nature.com/articles/s41594-023-00981-1
  105. https://journals.asm.org/doi/10.1128/mmbr.00046-10
  106. https://pmc.ncbi.nlm.nih.gov/articles/PMC6129461/
  107. https://academic.oup.com/g3journal/article/5/9/1889/6025248
  108. https://elifesciences.org/articles/34100
  109. https://pmc.ncbi.nlm.nih.gov/articles/PMC5887990/
  110. https://www.sciencedirect.com/science/article/abs/pii/S0022283625004516
  111. https://www.bmbreports.org/journal/download_pdf.php?doi=10.5483/BMBRep.2024-0191
  112. https://pmc.ncbi.nlm.nih.gov/articles/PMC3466053/
  113. https://pmc.ncbi.nlm.nih.gov/articles/PMC5041593/
  114. https://pmc.ncbi.nlm.nih.gov/articles/PMC4860946/
  115. https://pmc.ncbi.nlm.nih.gov/articles/PMC2677553/
  116. https://pubmed.ncbi.nlm.nih.gov/9489704/
  117. https://pmc.ncbi.nlm.nih.gov/articles/PMC262413/
  118. https://pmc.ncbi.nlm.nih.gov/articles/PMC7708052/
  119. https://pmc.ncbi.nlm.nih.gov/articles/PMC3069113/
  120. https://pmc.ncbi.nlm.nih.gov/articles/PMC5447479/
  121. https://pmc.ncbi.nlm.nih.gov/articles/PMC4641500/
  122. https://www.science.org/doi/10.1126/sciadv.ads4473
  123. https://pmc.ncbi.nlm.nih.gov/articles/PMC2593076/
  124. https://febs.onlinelibrary.wiley.com/doi/10.1111/j.1742-4658.2011.08282.x
  125. https://www.science.org/doi/10.1126/science.add8673
  126. https://pmc.ncbi.nlm.nih.gov/articles/PMC9891698/
  127. https://www.nature.com/articles/s41588-023-01476-x
  128. https://genesdev.cshlp.org/content/early/2024/03/13/gad.351278.123
  129. https://www.nature.com/articles/ncomms15616
  130. https://pmc.ncbi.nlm.nih.gov/articles/PMC2929501/
  131. https://pmc.ncbi.nlm.nih.gov/articles/PMC3365398/
  132. https://pmc.ncbi.nlm.nih.gov/articles/PMC8790285/
  133. https://pmc.ncbi.nlm.nih.gov/articles/PMC8092003/
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