TFIIB-related factor

The N-terminal half of one TFIIIB subunit, called BRF (for TFIIB-related factor), is similar In sequence to TFIIB (a Pol II factor). This similarity suggests that BRF and TFIIB perform a similar function in initiation, namely, to direct the polymerase to the correct start site. Once TFIIIB has bound to either a tRNA or SS-rRNA gene, Pol III can bind and Initiate transcription In the presence of ribonucleoside triphosphates. The BRF subunit of TFIIIB interacts specifically with one of the polymerase subunits unique to Pol III, accounting for Initiation by this specific nuclear RNA polymerase. 

TBP is a protein component of RNA pol I, II and III dependent initiation complexes, (a) TBP is part of the SL1 complex at RNA pol I dependent promoters, (b) TBP is bound to the TATA box in TATA+ RNA pol II-dependent promoters and to the initiator region (INR) in TATA-promoters and (c) TBP is associated with the TFIIB related factor (BRF) in initiation complexes at RNA pol III transcribed genes. The respective RNA polymerase core enzymes would also be present in these various initiation complexes. 

Colbert, T., and Hahn, S. (1992). A yeast TFIIB related factor involved in RNA polymerase III transcription. Genes Dev. 6, 1940-1949. 

Moir, R. D., Sethy-Coraci, I., Puglia, K., Librizzi, M. D., and Willis, I. M. (1997). A tetratricopeptide repeat mutation in yeast transcription factor IIIC131 (TFIIIC131) facilitates recruitment of TFIIB-related factor TFIIIB70. Mol. Cell. Biol. 17, 7119-7125. 

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