General activator of RNA polymerase which utilizes different TFIIIB complexes at structurally distinct promoters. The isoform 1 is involved in the transcription of tRNA, adenovirus VA1, 7SL and 5S RNA. Isoform 2 is required for transcription of the U6 promoter.
Both in yeast and mammalian cells, TFIIIB consists of three subunits: TFIIB-related Brf1, TATA-box binding protein (TBP), common also for the other two RNA polymerases, and Pol III-specific subunit, Bdp1 (Table 1).