Transcription factor II B

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 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: RNA polymerase II is recruited to DNA through the TFIIB B core and B ribbon. RNA polymerase II unwinds DNA, aided by the TFIIB B linker and B reader (open complex formation). RNA polymerase II selects a transcription start site, aided by the TFIIB B reader. RNA polymerase II forms the first phosphodiester bond. RNA polymerase II produces short abortive transcripts due to clashes between nascent RNA and the TFIIB B reader loop. Extension of nascent RNA to 12-13 nucleotides leads to ejection of TFIIB due to further clashes with TFIIB. 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.