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. TFIIIB comprises at least the TATA-binding protein (TBP) and the B-related factor 1 (BRF1/TFIIIB90). Interacts with BDP1 (PubMed:14592981). Interacts with MAF1 (PubMed:18377933). The disease is caused by variants affecting the gene represented in this entry. Belongs to the TFIIB family.