text: RNA polymerase III uses a very similar factor to TFIIB called Brf (TFIIB-related factor) which also contains a conserved zinc ribbon and C terminal core. However, the structure diverges in the more flexible linker region although Brf still contains highly conserved sequences in the same positions that the B reader and B linker are found. These conserved regions probably carry out similar functions as the domains in TFIIB. RNA polymerase I does not use a factor that is similar to TFIIB; however, it is thought that another unknown factor fulfils the same function.
  There is no direct homologue for TFIIB in bacterial systems but there are proteins that bind the bacterial polymerase in a similar manner with no sequence similarity. In particular the bacterial protein σ70 contains domains that bind the polymerase at the same points as the B-linker, B-ribbon and B-core. This is especially apparent in the σ 3 region and the region 4 linker which might stabilise the DNA in the polymerase active site.