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.

[9] 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.

The amino terminal B ribbon is located on dock domain of RNA polymerase II and extends in to the cleft towards the active site.

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.

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.

[14] The B reader loop is further thought to stabilise NTPs in the active site and, due to its flexibility, allow the nucleic acids to remain in contact during the early synthesis of the RNA molecule (i.e. stabilises the growing RNA-DNA hybrid) When the RNA transcript reaches 7 nucleotides long, transcription enters the elongation phase, the beginning of which is characterised by the collapsing of the DNA bubble and the ejection of TFIIB.

It has been suggested that the general transcription factor TFIIH could act as the kinase for this phosphorylation although more evidence is needed to support this.

[20] however, recent research has shown that a depletion in TFIIB is not lethal to cells and transcription levels are not significantly affected.

[21] This is because over 90% of mammalian promoters do not contain a BRE (B recognition element) or TATA box sequence which are required for TFIIB to bind.

Gene looping is reliant on the interaction between phosphorylated serine residues found on the C terminal domain of RNA polymerase II and polyadenylation factors.