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Ribosomal frameshift
Ribosomal frameshift
Ribosomal frameshift
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Ribosomal frameshifting, also known as translational frameshifting or translational recoding, is a biological phenomenon that occurs during translation that results in the production of multiple, unique proteins from a single mRNA.[1] The process can be programmed by the nucleotide sequence of the mRNA and is sometimes affected by the secondary, 3-dimensional mRNA structure.[2] It has been described mainly in viruses (especially retroviruses), retrotransposons and bacterial insertion elements, and also in some cellular genes.[3]

Small molecules, proteins, and nucleic acids have also been found to stimulate levels of frameshifting. In December 2023, it was reported that in vitro-transcribed (IVT) mRNAs in response to BNT162b2 (Pfizer–BioNTech) anti-COVID-19 vaccine caused ribosomal frameshifting.[4]

Process overview

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Proteins are translated by reading tri-nucleotides on the mRNA strand, also known as codons, from one end of the mRNA to the other (from the 5' to the 3' end) starting with the amino acid methionine as the start (initiation) codon AUG. Each codon is translated into a single amino acid. The code itself is considered degenerate, meaning that a particular amino acid can be specified by more than one codon. However, a shift of any number of nucleotides that is not divisible by 3 in the reading frame will cause subsequent codons to be read differently.[5] This effectively changes the ribosomal reading frame.

Sentence example

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In this example, the following sentence of three-letter words makes sense when read from the beginning: 

|Start|THE CAT AND THE MAN ARE FAT ...
|Start|123 123 123 123 123 123 123 ...

However, if the reading frame is shifted by one letter to between the T and H of the first word (effectively a +1 frameshift when considering the 0 position to be the initial position of T), 

T|Start|HEC ATA NDT HEM ANA REF AT...
-|Start|123 123 123 123 123 123 12...

then the sentence reads differently, making no sense.

DNA example

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In this example, the following sequence is a region of the human mitochondrial genome with the two overlapping genes MT-ATP8 and MT-ATP6. When read from the beginning, these codons make sense to a ribosome and can be translated into amino acids (AA) under the vertebrate mitochondrial code: 

|Start|AAC GAA AAT CTG TTC GCT TCA ...
|Start|123 123 123 123 123 123 123 ...
| AA  | N   E   N   L   F   A   S  ...

However, let's change the reading frame by starting one nucleotide downstream (effectively a "+1 frameshift" when considering the 0 position to be the initial position of A): 

A|Start|ACG AAA ATC TGT TCG CTT CA...
-|Start|123 123 123 123 123 123 12...
 | AA  | T   K   I   C   S   L    ...

Because of this +1 frameshifting, the DNA sequence is read differently. The different codon reading frame therefore yields different amino acids.

Effect

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In the case of a translating ribosome, a frameshift can either result in nonsense mutation, a premature stop codon after the frameshift, or the creation of a completely new protein after the frameshift. In the case where a frameshift results in nonsense, the nonsense-mediated mRNA decay (NMD) pathway may destroy the mRNA transcript, so frameshifting would serve as a method of regulating the expression level of the associated gene.[6]

If a novel or off-target protein is produced, it can trigger other unknown consequences.[4]

Function in viruses and eukaryotes

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In viruses this phenomenon may be programmed to occur at particular sites and allows the virus to encode multiple types of proteins from the same mRNA. Notable examples include HIV-1 (human immunodeficiency virus),[7] RSV (Rous sarcoma virus)[8] and the influenza virus (flu),[9] which all rely on frameshifting to create a proper ratio of 0-frame (normal translation) and "trans-frame" (encoded by frameshifted sequence) proteins. Its use in viruses is primarily for compacting more genetic information into a shorter amount of genetic material.

In eukaryotes it appears to play a role in regulating gene expression levels by generating premature stops and producing nonfunctional transcripts.[3][10]

Types of frameshifting

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The most common type of frameshifting is −1 frameshifting or programmed −1 ribosomal frameshifting (−1 PRF). Other, rarer types of frameshifting include +1 and −2 frameshifting.[2] −1 and +1 frameshifting are believed to be controlled by different mechanisms, which are discussed below. Both mechanisms are kinetically driven.

Programmed −1 ribosomal frameshifting

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Tandem slippage of 2 tRNAs at rous sarcoma virus slippery sequence. After the frameshift, new base pairings are correct at the first and second nucleotides but incorrect at wobble position. E, P, and A sites of the ribosome are indicated. Location of growing polypeptide chain is not indicated in image because there is not yet consensus on whether the −1 slip occurs before or after polypeptide is transferred from P-site tRNA to A-site tRNA (in this case from the Asn tRNA to the Leu tRNA).[8]

In −1 frameshifting, the ribosome slips back one nucleotide and continues translation in the −1 frame. There are typically three elements that comprise a −1 frameshift signal: a slippery sequence, a spacer region, and an RNA secondary structure. The slippery sequence fits a X_XXY_YYH motif, where XXX is any three identical nucleotides (though some exceptions occur), YYY typically represents UUU or AAA, and H is A, C or U. Because the structure of this motif contains 2 adjacent 3-nucleotide repeats it is believed that −1 frameshifting is described by a tandem slippage model, in which the ribosomal P-site tRNA anticodon re-pairs from XXY to XXX and the A-site anticodon re-pairs from YYH to YYY simultaneously. These new pairings are identical to the 0-frame pairings except at their third positions. This difference does not significantly disfavor anticodon binding because the third nucleotide in a codon, known as the wobble position, has weaker tRNA anticodon binding specificity than the first and second nucleotides.[2][11] In this model, the motif structure is explained by the fact that the first and second positions of the anticodons must be able to pair perfectly in both the 0 and −1 frames. Therefore, nucleotides 2 and 1 must be identical, and nucleotides 3 and 2 must also be identical, leading to a required sequence of 3 identical nucleotides for each tRNA that slips.[12]

+1 ribosomal frameshifting

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+1 frameshift occurs as ribosome and P-site tRNA pause to wait for arrival of rare arginine tRNA. The A-site codon in the new frame pairs to anticodon of more common glycine tRNA, and translation continues.[13]

The slippery sequence for a +1 frameshift signal does not have the same motif, and instead appears to function by pausing the ribosome at a sequence encoding a rare amino acid.[13] Ribosomes do not translate proteins at a steady rate, regardless of the sequence. Certain codons take longer to translate, because there are not equal amounts of tRNA of that particular codon in the cytosol.[14] Due to this lag, there exist in small sections of codons sequences that control the rate of ribosomal frameshifting. Specifically, the ribosome must pause to wait for the arrival of a rare tRNA, and this increases the kinetic favorability of the ribosome and its associated tRNA slipping into the new frame.[13][15] In this model, the change in reading frame is caused by a single tRNA slip rather than two.

Controlling mechanisms

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Ribosomal frameshifting may be controlled by mechanisms found in the mRNA sequence (cis-acting). This generally refers to a slippery sequence, an RNA secondary structure, or both. A −1 frameshift signal consists of both elements separated by a spacer region typically 5–9 nucleotides long.[2] Frameshifting may also be induced by other molecules which interact with the ribosome or the mRNA (trans-acting).

Frameshift signal elements

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This is a graphical representation of the HIV1 frameshift signal. A −1 frameshift in the slippery sequence region results in translation of the pol instead of the gag protein-coding region, or open reading frame (ORF). Both gag and pol proteins are required for reverse transcriptase, which is essential to HIV1 replication.[7]

Slippery sequence

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Slippery sequences can potentially make the reading ribosome "slip" and skip a number of nucleotides (usually only 1) and read a completely different frame thereafter. In programmed −1 ribosomal frameshifting, the slippery sequence fits a X_XXY_YYH motif, where XXX is any three identical nucleotides (though some exceptions occur), YYY typically represents UUU or AAA, and H is A, C or U. In the case of +1 frameshifting, the slippery sequence contains codons for which the corresponding tRNA is more rare, and the frameshift is favored because the codon in the new frame has a more common associated tRNA.[13] One example of a slippery sequence is the polyA on mRNA, which is known to induce ribosome slippage even in the absence of any other elements.[16]

RNA secondary structure

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Efficient ribosomal frameshifting generally requires the presence of an RNA secondary structure to enhance the effects of the slippery sequence.[12] The RNA structure (which can be a stem-loop or pseudoknot) is thought to pause the ribosome on the slippery site during translation, forcing it to relocate and continue replication from the −1 position. It is believed that this occurs because the structure physically blocks movement of the ribosome by becoming stuck in the ribosome mRNA tunnel.[2] This model is supported by the fact that strength of the pseudoknot has been positively correlated with the level of frameshifting for associated mRNA.[3][17]

Below are examples of predicted secondary structures for frameshift elements shown to stimulate frameshifting in a variety of organisms. The majority of the structures shown are stem-loops, with the exception of the ALIL (apical loop-internal loop) pseudoknot structure. In these images, the larger and incomplete circles of mRNA represent linear regions. The secondary "stem-loop" structures, where "stems" are formed by a region of mRNA base pairing with another region on the same strand, are shown protruding from the linear DNA. The linear region of the HIV ribosomal frameshift signal contains a highly conserved UUU UUU A slippery sequence; many of the other predicted structures contain candidates for slippery sequences as well.

The mRNA sequences in the images can be read according to a set of guidelines. While A, T, C, and G represent a particular nucleotide at a position, there are also letters that represent ambiguity which are used when more than one kind of nucleotide could occur at that position. The rules of the International Union of Pure and Applied Chemistry (IUPAC) are as follows:[18]

Symbol[18] Description Bases represented Complement
A Adenine A 1 T
C Cytosine C G
G Guanine G C
T Thymine T A
U Uracil U A
W Weak A T 2 W
S Strong C G S
M aMino A C K
K Keto G T M
R puRine A G R
Y pYrimidine C T Y
B not A (B comes after A) C G T 3 V
D not C (D comes after C) A G T H
H not G (H comes after G) A C T D
V not T (V comes after T and U) A C G B
N any Nucleotide (not a gap) A C G T 4 N
Z Zero 0 Z

These symbols are also valid for RNA, except with U (uracil) replacing T (thymine).[18]

Frameshift elements
Type Distribution Ref.
ALIL pseudoknot Bacteria [19]
Antizyme RNA frameshifting stimulation element Invertebrates [20]
Coronavirus frameshifting stimulation element Coronavirus [21]
DnaX ribosomal frameshifting element Eukaryota, bacteria [22]
HIV ribosomal frameshift signal Viruses
Insertion sequence IS1222 ribosomal frameshifting element Eukaryota, bacteria
Ribosomal frameshift Viruses

Trans-acting elements

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Small molecules, proteins, and nucleic acids have been found to stimulate levels of frameshifting. For example, the mechanism of a negative feedback loop in the polyamine synthesis pathway is based on polyamine levels stimulating an increase in +1 frameshifts, which results in production of an inhibitory enzyme. Certain proteins which are needed for codon recognition or which bind directly to the mRNA sequence have also been shown to modulate frameshifting levels. MicroRNA (miRNA) molecules may hybridize to an RNA secondary structure and affect its strength.[6]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Ribosomal frameshift

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Ribosomal frameshifting, also known as programmed ribosomal frameshifting (PRF), is a recoding mechanism during mRNA in which the slips by one or more , shifting its and enabling the synthesis of alternative protein isoforms from a single mRNA transcript. This process is facilitated by specific cis-acting mRNA elements, including slippery heptanucleotide sequences (such as XXXYYYZ for -1 frameshifting) that promote tRNA slippage and downstream stimulatory structures like RNA pseudoknots or stem-loops that induce ribosomal pausing to increase frameshift efficiency. Frameshifts can be -1 (backward by one ), +1 (forward by one), or rarer variants, with efficiencies ranging from less than 1% to over 50% depending on the biological context. In viruses, ribosomal frameshifting is a widespread strategy to maximize genomic efficiency, allowing the expression of multiple proteins from overlapping open reading frames (ORFs) within compact viral genomes. For instance, in retroviruses like HIV-1, -1 PRF at a slippery upstream of an RNA stem-loop produces a 1:20 ratio of to Gag-Pol polyproteins essential for viral maturation and . Similarly, coronaviruses such as employ -1 PRF to generate non-structural proteins critical for replication, with frameshift sites conserved across betacoronaviruses. This regulatory role makes PRF a promising target for antiviral therapies, as inhibiting frameshifting can disrupt stoichiometry and propagation. Beyond viruses, ribosomal frameshifting occurs in cellular organisms across all three domains of life—, , and eukaryotes—serving to fine-tune , expand diversity, and respond to environmental cues. In like , +1 PRF autoregulates 2 (RF2) synthesis by bypassing a in the prfB gene, maintaining optimal termination fidelity. Eukaryotic examples include +1 PRF in the antizyme (OAZ) genes of species from to humans, which controls levels by coupling frameshifting to substrate availability and influencing mRNA stability. Recent discoveries, such as +1 frameshifting in the human PLEKHM2 gene, further highlight its role in cellular diversity. Structural studies using cryo-EM and have revealed the dynamic ribosomal conformations and interactions underlying these events, highlighting evolutionary conservation and potential therapeutic implications in diseases linked to dysregulated translation.

Fundamentals

Definition and biological context

Ribosomal frameshifting is a translational recoding event in which the ribosome alters its reading frame on the mRNA during protein synthesis, shifting by one or more nucleotides (commonly +1 or -1) from the standard triplet codon sequence, thereby producing a distinct polypeptide from the same mRNA template. This programmed event is distinct from rare spontaneous frameshifts that occur as translational errors. It contrasts with the typical unidirectional progression of translation and allows for the synthesis of alternative protein products, often by fusing portions of overlapping open reading frames. In biological contexts, ribosomal frameshifting serves primarily as a for genome compaction and regulation, particularly in viruses where limited genetic material must encode multiple essential proteins. It is widely employed by retroviruses, coronaviruses, and other viral families to produce polyproteins like Gag-Pol fusions, enabling efficient replication without additional . Beyond viruses, frameshifting occurs in (e.g., in the dnaX gene of for and gamma subunit production), eukaryotes (e.g., in the Ty1 ), and retrotransposons such as mammalian Peg10, where it facilitates developmental regulation and error correction. Programmed frameshifting events typically occur at controlled efficiencies of 1–50%, balancing the production of standard and shifted proteins, while spontaneous frameshifts are rare (less than 10^{-5} per codon). Computational analyses have predicted that up to ~10% of eukaryotic mRNAs may be subject to -1 frameshifting mechanisms. The phenomenon was first described in the mid-1980s through studies on retroviruses, notably in , where -1 frameshifting was shown to express the pol gene from the gag-pol overlapping frame, revealing its role in viral enzyme production. This discovery, building on earlier observations of frameshift suppressors in from the –1970s, established ribosomal frameshifting as a deliberate recoding strategy rather than merely a translational error.

Comparison to standard translation

In standard translation, the ribosome decodes (mRNA) in a 5' to 3' direction by reading the nucleotide sequence in non-overlapping triplets known as codons, with each codon specifying a particular according to the . Transfer RNAs (tRNAs) serve as adaptors, carrying specific and base-pairing their anticodon loops with complementary mRNA codons in the ribosome's aminoacyl (A) site. is maintained through the peptidyl (, which holds the growing polypeptide chain attached to a tRNA, and mechanisms involving GTP hydrolysis that allow rejection of mismatched tRNA-codon pairs before formation, catalyzed by . This process ensures accurate, frame-preserving synthesis of a single protein from a defined (ORF), an uninterrupted sequence starting with an initiation codon (typically AUG) and ending at a , with error rates as low as 10^{-4} to 10^{-7} per codon. In contrast, ribosomal frameshifting disrupts this collinear decoding by causing the to slip along the mRNA by a number of that is not a multiple of three, such as one in a -1 frameshift, thereby switching to an alternative after the slip point. Unlike standard , where the remains fixed throughout the ORF, frameshifting does not occur spontaneously at appreciable rates in the canonical process, as the 's translocation maintains precise triplet alignment without slippage. This shift can result in the production of fusion proteins, where the N-terminal portion from the original frame merges with a C-terminal sequence from the new frame, or truncated proteins if the shift introduces a premature , fundamentally altering the downstream output from the same mRNA. A key prerequisite for understanding frameshifting involves the concept of open reading frames (ORFs), which in standard represent contiguous codon sequences that dictate a single polypeptide. Frameshifts enable the use of overlapping ORFs on the same mRNA, where the ribosome's programmed deviation allows access to a secondary ORF that would otherwise be out-of-frame and untranslatable in the default process. This overlap expands the coding potential of a single mRNA, permitting the synthesis of multiple protein products without requiring separate transcripts, a feature absent in standard where each ORF is translated independently and non-overlapping.

Molecular Mechanism

Core steps of the frameshift event

The core steps of the ribosomal frameshift event occur during the elongation phase of , when the ribosome temporarily alters its relative to the mRNA, diverging from the standard triplet decoding process. This recoding involves a coordinated sequence of molecular events that allow a subset of ribosomes to produce proteins from an alternative . The process begins with the ribosome pausing at a designated shift site within the mRNA. This pause arises from transient stalling during translocation, enabling the ribosome to dwell in a pre-translocation state where peptidyl-tRNA occupies the and is in the A-site. The pause duration, typically on the order of milliseconds to seconds, creates a window for frameshifting by slowing the rate of elongation compared to standard . Next, tRNA slippage or hopping takes place, primarily involving the tRNAs in the A- and s. In slippage, the anticodons of these tRNAs realign with the mRNA by shifting backward (for -1 frameshifts) or forward (for +1 frameshifts) by one , often facilitated by that permits wobble-like movement without dissociation. Hopping, less common, involves the tRNA briefly detaching and reattaching to a downstream codon, skipping one . These events are promoted by upstream stimulatory elements that enhance the probability of misalignment during the pause. Following slippage or hopping, the reinitiates in the new . The paused complex resolves through conformational adjustments, allowing the shifted tRNAs to stabilize in their new positions and resume chain elongation. This reinitiation is often coupled to the action of s, ensuring in the altered frame. Kinetic aspects of frameshifting are critical for its , which typically ranges from 1% to 50% in viral contexts, determining the proportion of ribosomes that undergo the event versus those that continue in the original frame. The process involves ribosomal conformational changes, such as intersubunit rotation (approximately 4–12°) and head swiveling (18–21°), which facilitate tRNA repositioning. GTP by G (EF-G) in prokaryotes or eukaryotic 2 (eEF2) in eukaryotes drives translocation and stabilizes the post-slippage state, with multiple hydrolysis cycles possible during extended pausing to modulate . Universal features of the frameshift event center on interactions at the A- and P-sites of the . The tRNAs in these sites adopt hybrid states ( and P/E), where the anticodon ends remain paired to mRNA while the CCA ends move, creating tension that aids slippage. These dynamics ensure that frameshifting can occur across diverse organisms and mRNA contexts, independent of specific sequence motifs.

Impact on protein synthesis

Ribosomal frameshifting alters the during , leading to the production of distinct protein products from a single mRNA molecule. In standard , ribosomes decode mRNA in a fixed frame to synthesize a single polypeptide, but frameshifting causes the ribosome to slip by one or more , often resulting in fusion proteins that combine sequences from overlapping open reading frames (ORFs). This recoding event enables the expression of multifunctional proteins, where the N-terminal portion from the zero frame fuses with a C-terminal extension from the shifted frame, enhancing proteomic diversity without additional genes. The ratio of frameshifted to non-shifted products is tightly controlled by the of the frameshift event, typically ranging from 1% to 80%, though commonly 5-40% in many systems, which dictates protein essential for cellular or viral function. Low-efficiency frameshifting ensures a majority of standard products while producing a minority of fusions, maintaining balanced expression levels; for instance, this ratio influences the relative abundance of structural versus enzymatic proteins. Frameshift is modulated by cis-acting elements and trans-factors, allowing dynamic adjustment of output in response to cellular conditions. Beyond product diversity, frameshifting exerts regulatory effects on mRNA stability and protein truncation. In eukaryotes, frameshifts introducing premature termination codons can activate (NMD), degrading aberrant mRNAs to prevent accumulation of faulty transcripts and conserve resources. Alternatively, frameshifting may generate truncated proteins with specialized roles, such as regulatory peptides that modulate downstream or signaling. Evolutionarily, this mechanism provides an advantage by maximizing coding capacity in compact genomes, enabling multifunctionality where one mRNA yields proteins with complementary or antagonistic activities, thus optimizing resource use and adaptability. Quantitative models highlight how frameshift efficiency impacts overall and replication dynamics. Kinetic partitioning frameworks describe frameshifting as a competition between standard translocation and slippage, with efficiency inversely correlating to replication rates—reductions in efficiency can diminish protein output by up to 90%, disrupting balanced expression and impairing processes like viral propagation or cellular . These models incorporate thermodynamic stability of structures and ribosomal pausing rates, predicting how perturbations alter stoichiometric ratios and downstream effects on fidelity.

Types of Frameshifting

Programmed -1 frameshifting

Programmed -1 frameshifting involves the slipping backward by one during , altering the to produce an extended or alternative protein from a single mRNA molecule. This recoding event allows for the regulated expression of multiple polypeptides, optimizing utilization in resource-limited organisms. It is the predominant form of programmed frameshifting, occurring in known viral cases and select bacterial genes, where it controls the stoichiometric balance of protein products essential for replication or cellular function. The mechanism relies on simultaneous slippage of the peptidyl-tRNA in the and the in the A site along a slippery heptamer in the mRNA, commonly conforming to the motif XXXYYYZ (where X denotes any and Y a ). This slippage is triggered during the translocation step of elongation, often under conditions of limited availability, and is enhanced by ribosomal pausing. Efficiency, typically ranging from 1% to 50%, is finely tuned by a downstream cis-acting RNA structure, such as a , which mechanically restrains the , promoting the backward shift while minimizing errors in standard . Seminal studies on the HIV-1 gag-pol overlap identified this slippery and its role in viral polyprotein processing. The first bacterial example of this process was identified in 1990 within the Escherichia coli dnaX gene, where -1 frameshifting generates the shorter γ subunit of III from the longer τ subunit , ensuring appropriate holoenzyme assembly. Recent cryo-EM structural models have elucidated the dynamics, revealing ribosomal twisting and a strained conformation upon engagement with the frameshift stimulatory element, such as in , where the imposes torsional restraint to facilitate pausing and slippage. These insights highlight how the ribosome's conformational changes underpin the precision of programmed -1 frameshifting.

+1 frameshifting

+1 ribosomal frameshifting involves the advancing by one in the 5' to 3' direction during , resulting in a shift from the zero frame to the +1 and the production of chimeric proteins from overlapping open reading frames (ORFs). Unlike the more prevalent -1 frameshifting, which occurs in numerous viral and cellular systems, +1 frameshifting is rarer, with spontaneous rates around 10^{-5} per codon and documented programmed instances in across bacteria and eukaryotes. A notable bacterial example is the +1 PRF in the E. coli prfB , which autoregulates 2 (RF2) synthesis by bypassing a . Notable natural examples include the Saccharomyces cerevisiae antizyme OAZ1, where +1 frameshifting regulates biosynthesis by controlling antizyme protein levels essential for . The mechanism of +1 frameshifting typically lacks a slippery sequence, distinguishing it from -1 events, and instead relies on ribosome stalling or pausing, often induced by rare codons, codon repeats, or specific sequence contexts that promote tRNA dissociation. In OAZ1, stimulate the frameshift by enhancing the dissociation of the tRNA from its codon, allowing it to re-pair with the +1 frame codon (e.g., at a CCC-U site), which bypasses a premature termination codon and fuses upstream and downstream ORFs to produce full-length functional antizyme. This process can involve near-cognate tRNA pairing or E-site interactions that facilitate the forward slip, with efficiencies modulated by cellular conditions such as levels or translational stress. Emerging contexts highlight +1 frameshifting in synthetic mRNAs, particularly those modified with (m1Ψ) for enhanced stability in therapeutics. A 2023 study demonstrated that full substitution of uridines with m1Ψ in mRNA, as used in vaccines like BNT162b2, induces +1 frameshifting at slippery sequences due to slowed elongation and ribosome stalling, with efficiencies reaching approximately 8% relative to in-frame in vitro. This leads to the production of aberrant chimeric spike proteins, eliciting off-target T-cell immune responses in vaccinated mice and humans, as evidenced by significant IFNγ ELISpot responses to +1 frameshifted peptides (P = 0.0005 in mice; P = 0.0233 in humans). Such findings underscore potential safety concerns for m1Ψ-modified mRNAs, including unintended from frameshifted products identified via .

Regulatory Elements

Cis-acting signals

Cis-acting signals in ribosomal frameshifting are intrinsic mRNA elements that direct the to alter its without requiring external protein factors. These signals typically comprise a slippery sequence and a downstream secondary structure, which together promote tRNA slippage and ribosomal pausing, respectively. In most cases, both elements are essential for efficient frameshifting, particularly in programmed - events prevalent in viral genomes. Slippery sequences are heptanucleotide motifs that enable simultaneous backward slippage of the P-site and A-site tRNAs by one nucleotide during elongation. For -1 frameshifting, the consensus motif is X XXY YYZ (where the spaces denote codon boundaries in the zero frame, X is A or U, Y is A or U, and Z is A, C, or G), which positions the tRNAs on slippery codons like AAA and AAG to facilitate realignment in the -1 frame. A representative example is the A AAA AAG sequence in the Escherichia coli dnaX gene, where slippage allows production of short and long tau subunits. These sequences alone induce low-level frameshifting (approximately 0.1-1%), but their efficacy depends on precise positioning 5-9 nucleotides upstream of the secondary structure to align with the ribosomal A site during pausing. Downstream RNA secondary structures, most commonly H-type pseudoknots but occasionally stem-loops, cause the to stall as it encounters the mRNA, increasing the time available for tRNA slippage at the upstream slippery sequence. Pseudoknots form when the single-stranded loop of a stem-loop pairs with a complementary downstream sequence, creating a stable tertiary structure that mechanically resists unwinding by the advancing . The thermodynamic stability of these pseudoknots, quantified by free energy changes (ΔG) typically ranging from -15 to -25 kcal/mol under physiological conditions, positively correlates with frameshifting efficiency; structures with more negative ΔG values prolong pausing and thus enhance slippage rates. For instance, the pseudoknot in the mouse mammary tumor virus (MMTV) exhibits a ΔG of approximately -15.4 kcal/mol, contributing to 20-30% frameshifting efficiency. Stem-loops can substitute in some contexts but generally yield lower efficiency due to reduced stability compared to pseudoknots. The synergistic integration of these signals is a hallmark of efficient -1 frameshifting, with nearly all characterized viral examples requiring both elements to achieve biologically relevant efficiencies of 5-50%, far exceeding the basal slippage rate. This combination ensures precise control over production, such as the Gag-Pol polyprotein in retroviruses. Evolutionarily, these cis-acting motifs are highly conserved across viral genomes within families like Retroviridae and , preserving core slippery consensus and topologies despite sequence divergence to maintain frameshift regulation under selective pressure.

Trans-acting factors

Trans-acting factors are extrinsic cellular components, such as proteins and small molecules, that modulate the efficiency of ribosomal frameshifting by interacting with the or mRNA, often in concert with cis-acting signals to fine-tune translation under specific conditions. These factors can enhance pausing, slippage, or translocation inhibition, thereby influencing the proportion of frameshifted products. For instance, in , the ribosomal protein L9 (bL9) plays a key role in maintaining translational fidelity by suppressing -1 frameshifting; its absence leads to a approximately 2-fold increase in frameshifting efficiency at sites like the dnaX , where it also affects the spacing of colliding ribosomes to prevent slippage. Similarly, in eukaryotes, the eRF1, often acting synergistically with eRF3, inhibits +1 frameshifting in contexts like antizyme synthesis, reducing efficiency in cell-free systems when added exogenously. Other protein factors include ribosome-associated molecular chaperones, which exert specific effects on frameshifting. In , the ribosome-tethered chaperones Ssb1p/Ssb2p and the Ssz1p/Zuo1p complex (RAC) inhibit programmed -1 frameshifting without affecting +1 events, as demonstrated in dual-luciferase reporter assays where their deletion reduced -1 PRF efficiency and impaired maintenance of killer viruses. In viral systems, host proteins like poly(C)-binding proteins (PCBP) can trans-activate frameshifting; for example, PCBP forms a complex with viral nsp1β to enhance -1 frameshifting efficiency by up to 3-fold at slippery sequences. These protein factors often display context-dependency, boosting frameshift efficiency 2- to 5-fold when cis elements like Shine-Dalgarno sequences are present, as seen in bacterial dnaX where high loading decreases -1 frameshifting from ~70% to ~45%, potentially as a mechanism during stress. Small molecules, particularly antibiotics, represent another class of trans-acting factors that induce frameshifting by directly perturbing ribosomal function. antibiotics like erythromycin bind within the nascent exit tunnel, blocking translocation and promoting -1 frameshifting at specific sites; in , this leads to derepression of the ermC resistance gene by causing ribosome stalling in the leader , which allows continued past a stop codon and expression of the methyltransferase during antibiotic exposure. Such induction is modulated by interactions with cis signals, highlighting how trans factors can exploit ribosomal pausing to regulate in bacterial stress responses, including those involving essential genes like dnaX for regulation.

Biological Roles

In viruses

Ribosomal frameshifting plays a crucial role in by enabling genome compaction, allowing viruses to encode multiple proteins from a single (ORF) despite their limited sizes. In retroviruses such as HIV-1, a -1 frameshift event during of the gag-pol mRNA produces the Gag-Pol polyprotein at an efficiency of 5-10%, ensuring an optimal Gag:Gag-Pol ratio essential for virion assembly and . This mechanism overlaps the gag and pol ORFs, maximizing coding capacity without expanding the , a strategy conserved across the Retroviridae family. In coronaviruses, including , a similar -1 frameshift regulates expression of the ORF1ab polyprotein, which is critical for generating viral replicase enzymes. The frameshift occurs at 14-27% efficiency, balancing production of the upstream ORF1a and downstream ORF1b products to support genome replication. This compaction allows the virus to produce two polyproteins from one continuous ORF, enhancing efficiency in the family. Additionally, some strains () employ +1 frameshifting at approximately 1% efficiency within segment 3, generating the PA-X to modulate host responses while conserving genomic space. From an evolutionary perspective, ribosomal frameshifting represents an adaptive strategy for viruses with small genomes, enabling them to encode essential multifunctional proteins without additional genetic material. This recoding mechanism is under strong selective pressure, as evidenced by the conservation of frameshift signals across viral isolates despite high RNA mutation rates that could disrupt slippery sequences or stimulatory elements. In retroviruses and coronaviruses, variations in these sites correlate with fitness trade-offs, underscoring frameshifting's role in long-term viral to host environments.

In cellular organisms

Ribosomal frameshifting in cellular organisms serves as a precise mechanism for regulating gene expression, enabling the production of protein isoforms that balance essential cellular processes without relying on alternative promoters or splicing. In bacteria, this recoding event is exemplified by the dnaX gene in Escherichia coli, where a programmed -1 frameshift occurs during translation of the DNA polymerase III holoenzyme subunits. This frameshift, occurring with approximately 50% efficiency at a slippery sequence (A AAA AAG), truncates the full-length τ subunit (71 kDa) into the shorter γ subunit (47 kDa), maintaining a 1:1 ratio critical for replicative DNA polymerase function and genomic stability. The γ subunit retains the core catalytic domains but lacks τ's additional regions for processivity clamp loading, thus optimizing holoenzyme assembly. Another bacterial instance involves the prfB gene, which encodes release factor 2 (RF2) and utilizes a +1 frameshift at a CUU-UGA motif to autoregulate its expression. Under normal conditions, the frameshift efficiency is around 50%, producing full-length RF2 only when needed to terminate translation at UGA or UAA codons; the out-of-frame product lacks RF2 function and is degraded. This mechanism intensifies during metabolic stress, such as excess glucose, where increased frameshifting elevates RF2 levels to enhance translational fidelity and promote , preventing imbalance. In eukaryotes, programmed frameshifting similarly fine-tunes regulatory pathways, as seen in the antizyme (OAZ1) gene across species including (Saccharomyces cerevisiae) and humans. A +1 frameshift, stimulated by levels, occurs at a UCC-UGA site, with efficiency increasing under high conditions, yielding full-length antizyme that binds and inhibits (ODC), the rate-limiting enzyme in biosynthesis. This autoregulatory loop prevents excess, which could otherwise disrupt and proliferation; in , it also targets ODC for ubiquitin-independent proteasomal degradation, ensuring . Although specific -1 frameshifting cases in human cellular genes are less documented, broader surveys indicate its role in producing multifunctional proteins, such as in placental development via the retrotransposon-derived PEG10 gene. Beyond these examples, ribosomal frameshifting contributes to cellular by triggering of aberrant mRNAs, stress responses through adjusted protein stoichiometries during environmental challenges, and developmental processes by modulating isoform ratios in tissue-specific contexts. Computational analyses suggest that potential frameshift signals occur more frequently than expected by chance in eukaryotic genes, underscoring their selective but widespread utility in non-viral regulation.

Examples and Case Studies

Viral systems

Ribosomal frameshifting was first discovered in the (RSV) in 1985, where a -1 frameshift event at the gag-pol junction produces the Gag-Pol polyprotein essential for . The frameshift occurs via tRNA slippage on a slippery sequence (A AAA AAC), stimulated by a downstream stem-loop structure, with an efficiency of approximately 5-10% to balance Gag and Gag-Pol production. This mechanism ensures the correct stoichiometric ratio for virion assembly and reverse transcription. In human type 1 (HIV-1), programmed - frameshifting at the gag-pol overlap generates the -Pol , which includes the necessary for viral genome replication. The event is directed by a slippery heptanucleotide sequence U UUU UUA, followed 6-8 nucleotides downstream by an RNA pseudoknot that stalls the to promote slippage. Frameshifting efficiency is tightly regulated at approximately 5%, optimizing the Gag:Gag-Pol ratio for particle maturation and . Severe acute respiratory syndrome coronavirus 2 () employs -1 frameshifting to express the replicase polyprotein from overlapping open reading frames 1a and 1b (ORF1a/b). The slippery sequence U UUA AAC, paired with a complex three-stemmed stimulator, induces frameshifting with an efficiency of approximately 25-28%, allowing production of the nsP12 fused to upstream components for activity. This regulated ratio of ORF1a to ORF1ab products is critical for viral genome replication and immune evasion. Barley yellow dwarf virus (BYDV), a plant luteovirus, utilizes -1 frameshifting to fuse open reading frames 1 and 2, producing a polyprotein that includes the for viral propagation. The frameshift is stimulated by a structure interacting with distant elements, achieving an efficiency of approximately 1-6% to coordinate and replicase expression. This mechanism exemplifies frameshifting adaptation in plant RNA viruses for efficient genome utilization.

Non-viral systems

In non-viral systems, programmed ribosomal frameshifting serves regulatory roles in cellular organisms, enabling the production of protein isoforms essential for processes such as DNA replication, polyamine homeostasis, and stress adaptation. These events occur at specific efficiencies to balance the synthesis of full-length and truncated proteins, often in response to cellular conditions. In bacteria, a prominent example is the dnaX gene of Escherichia coli, which encodes the tau (τ) and gamma (γ) subunits of DNA polymerase III holoenzyme. The full-length τ subunit (71 kDa) is produced by standard translation, while the shorter γ subunit (47 kDa) results from a programmed -1 frameshift near the end of the coding sequence, with an efficiency of approximately 80%. This frameshift is facilitated by a slippery heptamer sequence (A AAA AAG) and an adjacent stem-loop structure acting as an attenuator, which pauses the ribosome to promote slippage. The balanced production of τ and γ ensures proper holoenzyme assembly, as τ dimerizes the core polymerase while γ does not, regulating replicative processivity. In eukaryotes, the ornithine decarboxylase antizyme 1 (OAZ1) gene in exemplifies +1 frameshifting for autoregulation of synthesis. The Oaz1 mRNA contains an internal within a slippery (UCC UGA), where high intracellular levels stimulate a +1 shift, allowing readthrough to produce full-length antizyme protein. This frameshift efficiency increases up to 50-fold with elevated , enabling antizyme to bind and inhibit (ODC), the rate-limiting enzyme in , while also targeting ODC for proteasomal degradation. The mechanism involves -induced conformational changes in the mRNA, ensuring feedback control to prevent excess. Oaz1 follows the conserved mammalian pattern, with the +1 codon embedded in the antizyme coding . Other examples include -1 frameshifting in the yeast Ty1 retrotransposon, which fuses the gag (TYA) and pol (TYB) open reading frames to produce a Gag-Pol polyprotein containing integrase. This occurs at a slippery site (CUU AGG) with an efficiency of about 40%, regulated by a 14-nucleotide sequence that promotes ribosomal slippage via tRNA interactions, essential for retrotransposition and virus-like particle maturation. In the plant , ribosomal frameshifting contributes to stress responses, where diphthamide biosynthesis pathway mutants (e.g., dph1) exhibit elevated -1 frameshift errors under abiotic stresses like or salt, affecting translational fidelity in stress-related genes. tRNA modifications induced by stress, such as queuosine, suppress these errors to maintain growth and , highlighting frameshifting's role in environmental resilience.

Recent Developments

Structural and mechanistic insights

Recent advances in cryo-electron microscopy (cryo-EM) have provided atomic-level insights into ribosomal frameshifting mechanisms, particularly through structures of ribosome-mRNA complexes captured during -1 and +1 shift events. A landmark 2021 study resolved the mammalian 80S ribosome stalled at the SARS-CoV-2 frameshift stimulatory element (FSE) at 2.2 Å in the core region, revealing how the H-type pseudoknot resists unfolding in the mRNA entry channel, generating mechanical tension that promotes backward slippage of P-site tRNA^Phe^ along the slippery sequence from U_UUA_AAC to UUU_AAA_C. This structure highlights interactions between the pseudoknot's Stem 1 and 2 with ribosomal proteins uS3, uS5, and eS30, as well as 18S rRNA, stabilizing the paused conformation essential for -1 frameshifting. Subsequent refinements in 2023 integrated this cryo-EM model with small-angle X-ray scattering (SAXS)-driven molecular dynamics simulations, confirming a bent pseudoknot conformation on the ribosome that enhances frameshift efficiency by optimizing tRNA-mRNA register shifts. For +1 frameshifting, high-resolution cryo-EM structures of bacterial 70S ribosomes from 2021 captured the process during EF-G-catalyzed translocation at resolutions of 3.2–3.3 Å, delineating intermediate states where an open 30S subunit and wobble base pairing (cmo⁵U34-C3) predispose tRNA^Pro^ for forward slippage into the +1 frame. These snapshots illustrate how stabilizes the mid-translocation complex, with the mRNA bulge (C1) facilitating the shift from CCC to C_CA, completing the +1 register change post-translocation. Building on this, mechanistic models emphasize the ribosome's intrinsic activity in unwinding stems during translocation; a 2021 study demonstrated that simulated ribosomal pausing at the slippery sequence enhances formation and resistance to helicase-mediated unfolding, prolonging the window for tRNA slippage in -1 events. Molecular dynamics simulations have further elucidated pause dynamics in +1 frameshifting, revealing how transient ribosomal stalling at rare codons or structured mRNAs extends translocation times, allowing to bias the tRNA-mRNA pairing toward the +1 frame. Recent genomic surveys from 2024–2025 across over 12,000 bacterial genomes confirm the evolutionary conservation of prfB frameshift sites, with core motifs (Shine-Dalgarno-like sequence, slippery heptamer, and TGA stop) preserved in 64% of taxa, suggesting an ancestral role in RF2 autoregulation that persists across phyla despite losses in high-TGA-utilizing groups like . In 2025, cryo-EM and biochemical studies revealed a +1 frameshifting event in the PLEKHM2 gene, accessing an overlapping (ORF) and expanding the known roles of programmed frameshifting in eukaryotic proteome diversity.

Therapeutic and applied aspects

Ribosomal frameshifting has emerged as a double-edged sword in design, particularly with the incorporation of (m1Ψ) modifications to enhance stability and translation efficiency. A 2023 study demonstrated that m1Ψ in the Pfizer-BioNTech BNT162b2 induces +1 ribosomal frameshifting at approximately 8% efficiency during translation of the mRNA, leading to the production of off-target polypeptides. These frameshifted products elicit unintended T-cell immune responses, as evidenced by IFNγ assays showing significant reactivity in vaccinated mice (P = 0.0005) and s (P = 0.0233 compared to ChAdOx1 nCoV-19 recipients), raising concerns about potential or toxicity from off-target immunogens. To mitigate this, researchers recommend redesigning mRNA sequences by mutating slippery sites to minimize frameshifting without compromising . In antiviral , inhibiting -1 ribosomal frameshifting offers a promising to disrupt synthesis, particularly in retroviruses like HIV-1, where frameshifting regulates the Gag-to-Gag-Pol ratio essential for virion assembly. Small molecules identified through resin-bound dynamic combinatorial library screening bind specifically to the HIV-1 frameshift stimulatory signal (FSS) RNA structure—a stem-loop —stabilizing it in a non-frameshifting conformation and reducing frameshift efficiency by up to 50%, thereby decreasing viral infectivity in cell-based assays against both wild-type and drug-resistant strains. For instance, these compounds exhibit high affinity (Kd ~10-100 nM) and selectivity for the HIV-1 FSS over human , demonstrating low and paving the way for broader antiviral applications. A 2025 study identified the host stem-loop binding protein (SLBP) as a promoter of -1 frameshifting, suggesting it as a novel druggable target to inhibit . Frameshifting modulation holds potential in for controlled transgene expression. leverages programmed ribosomal frameshifting to achieve precise control over protein from a single mRNA, enabling the production of multiple isoforms in defined ratios for complex pathway engineering. For example, standardized genetic parts incorporating -1 frameshift elements allow tunable labeling, where frameshifting bypasses stop codons to generate a fixed proportion (e.g., 5-20%) of extended polypeptides, facilitating applications in metabolic engineering and design. However, challenges persist in accurately predicting frameshift efficiency, as it depends on interdependent factors like mRNA secondary structure stability and slippery sequence context, with current models showing only moderate correlation (R² ~0.6) to experimental outcomes and requiring high-throughput assays for optimization. These limitations underscore the need for advanced computational tools to enhance design predictability in synthetic circuits.

References

  1. https://www.nature.com/articles/s41467-020-16961-8
  2. https://pmc.ncbi.nlm.nih.gov/articles/PMC1885200/
  3. https://www.annualreviews.org/doi/full/10.1146/annurev-virology-111821-120646
  4. https://bmcmicrobiol.biomedcentral.com/articles/10.1186/s12866-022-02503-3
  5. https://pmc.ncbi.nlm.nih.gov/articles/PMC12551717/
  6. https://www.annualreviews.org/doi/full/10.1146/annurev-biochem-060614-033742
  7. https://www.cell.com/trends/biochemical-sciences/fulltext/S0968-0004(15)00044-4
  8. https://www.science.org/doi/10.1126/science.2416054
  9. https://pmc.ncbi.nlm.nih.gov/articles/PMC8246090/
  10. https://pmc.ncbi.nlm.nih.gov/articles/PMC2650885/
  11. https://www.ncbi.nlm.nih.gov/books/NBK9849/
  12. https://pmc.ncbi.nlm.nih.gov/articles/PMC7126180/
  13. https://pmc.ncbi.nlm.nih.gov/articles/PMC3500957/
  14. https://pmc.ncbi.nlm.nih.gov/articles/PMC3419312/
  15. https://doi.org/10.1016/j.csbj.2021.06.015
  16. https://www.nature.com/articles/331280a0
  17. https://pmc.ncbi.nlm.nih.gov/articles/PMC6771820/
  18. https://pmc.ncbi.nlm.nih.gov/articles/PMC330589/
  19. https://www.nature.com/articles/s41594-021-00653-y
  20. https://pmc.ncbi.nlm.nih.gov/articles/PMC10954444/
  21. https://pmc.ncbi.nlm.nih.gov/articles/PMC535087/
  22. https://www.mdpi.com/1422-0067/23/21/12972
  23. https://www.nature.com/articles/s41586-023-06800-3
  24. https://doi.org/10.1006/jmbi.2000.3668
  25. https://doi.org/10.1128/jvi.66.8.5147-5151.1992
  26. https://pmc.ncbi.nlm.nih.gov/articles/PMC310776/
  27. https://www.pnas.org/doi/10.1073/pnas.1910613116
  28. https://pubmed.ncbi.nlm.nih.gov/14530443/
  29. https://pubmed.ncbi.nlm.nih.gov/16607023/
  30. https://pubmed.ncbi.nlm.nih.gov/27257056/
  31. https://pubmed.ncbi.nlm.nih.gov/25850333/
  32. https://pmc.ncbi.nlm.nih.gov/articles/PMC2575084/
  33. https://pmc.ncbi.nlm.nih.gov/articles/PMC7114087/
  34. https://pmc.ncbi.nlm.nih.gov/articles/PMC3498833/
  35. https://www.pnas.org/doi/10.1073/pnas.2221324120
  36. https://doi.org/10.1006/jmbi.1997.1213
  37. https://academic.oup.com/nar/article/18/7/1725/1096655
  38. https://www.pnas.org/doi/10.1073/pnas.2013543117
  39. https://doi.org/10.1093/emboj/cdg561
  40. https://pubmed.ncbi.nlm.nih.gov/2416054/
  41. https://europepmc.org/article/med/2846182
  42. https://pmc.ncbi.nlm.nih.gov/articles/PMC3988561/
  43. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0122176
  44. https://pmc.ncbi.nlm.nih.gov/articles/PMC8168617/
  45. https://www.jbc.org/article/S0021-9258(17)50110-6/fulltext
  46. https://pmc.ncbi.nlm.nih.gov/articles/PMC123222/
  47. https://www.sciencedirect.com/science/article/abs/pii/S0022283601948016
  48. https://pmc.ncbi.nlm.nih.gov/articles/PMC48637/
  49. https://genesdev.cshlp.org/content/6/3/511.full.pdf
  50. https://www.uniprot.org/uniprotkb/P54369/entry
  51. https://pmc.ncbi.nlm.nih.gov/articles/PMC7133245/
  52. https://www.nature.com/articles/s41467-022-31712-7
  53. https://www.science.org/doi/10.1126/science.abf3546
  54. https://academic.oup.com/nar/article/51/20/11332/7306679
  55. https://www.nature.com/articles/s41467-021-24911-1
  56. https://academic.oup.com/nar/article/49/12/6941/6308499
  57. https://journals.asm.org/doi/10.1128/mbio.01055-25
  58. https://www.science.org/doi/10.1126/sciadv.ady1742
  59. https://pubs.acs.org/doi/10.1021/acs.accounts.1c00316
  60. https://www.nature.com/articles/s41392-025-02277-w
  61. https://www.tandfonline.com/doi/abs/10.1080/21690731.2015.1112458
  62. https://pubmed.ncbi.nlm.nih.gov/18367782/
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