Domains of the Brf component of RNA polymerase III transcription factor IIIB (TFIIIB): Functions in assembly of TFIIIB-DNA complexes and recruitment of RNA polymerase to the promoter

Saccharomyces cerevisiae transcription factor IIIB (TFIIIB) is composed of three subunits: the TATA-binding protein, the TFIIB-related protein Brf, and B '', TFIIIB, which is brought to RNA polymerase III-transcribed genes indirectly through interaction with DNA-bound TFIIIC or directly through DNA recognition by the TATA-binding protein, in turn recruits RNA polymerase III to the promoter. N-terminally deleted derivatives of Brf have been examined for their ability to interact with DNA-bound TFIIIC and with the other components of TFIIIB and for participation in transcription, Brf(165-596), lacking 164 N-proximal TFIIB-homologous amino acids, is competent to participate in the assembly of TFIIIB-DNA complexes and in TFIIIC-independent transcription, Even deletion of the entire TFIIB-homologous half of the protein, as in Brf(317-596) and Brf(352-596), allows some interaction with DNA-bound TBP and with the B '' component of TFIIIB to be retained, The function of Brf(165-596) in transcription has also been examined in the context of B '' with small internal deletions. The ability of Brf with this sizable N-terminal segment deleted to function in TFIIIC-independent transcription requires segments of B '' that are individually indispensable although required on an either/or basis, in the context of complete Brf. These findings suggest a functional complementarity and reciprocity between the Brf and B '' components of TFIIIB.