Transcription factor II B (TFIIB) is a general transcription factor that is involved in the formation of the RNA polymerase II preinitiation complex (PIC) and aids in stimulating transcription initiation. TFIIB is localised to the nucleus and provides a platform for PIC formation by binding and stabilising the DNA-TBP (TATA-binding protein) complex and by recruiting RNA polymerase II and other transcription factors. It is encoded by the TFIIB gene, and is homologous to archaeal transcription factor B and analogous to bacterial sigma factors.

TFIIB is a single 33kDa polypeptide consisting of 316 amino acids. TFIIB is made up of four functional regions: the C-terminal core domain; the B linker; the B reader and the amino terminal zinc ribbon.

TFIIB makes protein-protein interactions with the TATA-binding protein (TBP) subunit of transcription factor IID, and the RPB1 subunit of RNA polymerase II.

TFIIB makes sequence-specific protein-DNA interactions with the B recognition element (BRE), a promoter element flanking the TATA element.

There are six steps in the mechanism of TFIIB action in the formation of the PIC and transcription initiation:

Each of the functional regions of TFIIB interacts with different parts of RNA polymerase II. The amino terminal B ribbon is located on dock domain of RNA polymerase II and extends in to the cleft towards the active site. Extending the B ribbon is the B reader that extends via the RNA exit tunnel to the binding site of the DNA-RNA hybrid and towards the active site. The B linker is the region between the B reader and the B core that is found in the cleft of RNA polymerase II and continues by the rudder and the clamp coiled-coil until it reaches the C terminal B core that is found above the wall of RNA polymerase II. The B reader and the B linker consist of highly conserved residues that are positioned through the RNA polymerase II tunnel towards the active site and ensure tight binding, without these key residues dissociation would occur. These two domains are also thought to adjust the position of some of the more flexible areas of RNA polymerase II to allow for the precise positioning of the DNA and allowing the addition of the new NTPs onto the nascent RNA chain. Upon binding RNA polymerase II, the B reader and B linker cause slight repositioning of the protrusion domain of RNA polymerase II which allows an essential second magnesium ion to bind in the active site. It forms a beta sheet and an ordered loop that helps with the stability of the structure when transcription is initiated.

The open and closed conformations refer to the state of the DNA and whether the template strand has been separated from the non-template strand within the PIC. The place at which the DNA opens to form the bubble lies above a tunnel that is lined by the B-core, B-linker and B-reader as well as parts of RNA polymerase II. The B linker is found directly aligned with the point at which the DNA opens and in the open complex it is found between the two DNA strands, suggesting that it has a role in promoter melting, but it does not have a role in the catalytic RNA synthesis. Although TFIIB keeps a similar structure in both conformations some of the intramolecular interactions between the core and the B reader are disrupted upon DNA opening.

After DNA melting the transcription initiator (Inr) must be located on the DNA so the TSS can be identified by the RNA polymerase II and transcription can begin. This is done by passing the DNA through the 'template tunnel' and the DNA is scanned, looking for the Inr and placing it in a position that ensures the transcription start site is located in the correct place by the RNA polymerase active site. The B reader of TFIIB is found in the template tunnel and is important in locating the Inr, mutations in the B reader cause the TSS to change and therefore incorrect transcription to occur (although PIC formation and DNA melting still take place). Yeast are a particularly good example of this alignment as the yeast Inr motif has a strictly conserved A residue at position 28 and in the open complex model a complementary T residue can be found in the B reader helix. When this T residue is mutated, transcription was significantly less effective emphasizing the role of the B reader.