A post-recruitment function for the RNA polymerase III transcription–initiation factor IIIB

Transcription factor (TF) IIIB, which directs RNA polymerase (pol) III to its promoters, is made up of three components: the TATA box-binding protein, the TFIIB-related Brf, and the pol III-specific B′′. Certain mutations in Saccharomyces cerevisiae Brf and B′′ retain TFIIIB transcription factor activity with supercoiled DNA but are inactive with linear duplex DNA. Further analysis shows that these inactive TFIIIB–DNA complexes bind pol III and position it appropriately over the transcriptional start site but do not form DNA strand-separated open promoter complexes. It is proposed that the normal function of TFIIIB combines pol III recruitment with an active role in a subsequent step of transcriptional initiation leading to promoter opening.