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. This is the function of BRF1 (BRF1 general transcription factor IIIB subunit, Ensembl gene identifier ENSG00000185024).