Two essential components of the Saccharomyces cerevisiae transcription factor TFIIIB: Transcription and DNA-binding properties

RNA polyrnerase III transcription factor TFIIIB from Saccharomyces cerevisiae contains at least two polypeptides, with apparent masses of 90 and 70 kDa, that were previously identified by photocrosslinking to DNA. It is shown here that TFIIIB can be chromatographically separated into two components, each of which is required for efficient tRNA gene transcription. DNA-protein photocrosslinking experiments show these two components separately contain the 90-and 70-kDa TFIIIB-specific polypeptides. The 70-kDa component forms a heparin-sensitive complex with transcription factor TFIIIC and DNA, stabilizes TFIIIC interaction with the tRNA gene promoter elements, and protects against DNase I digestion in the 3′ portion of the upstream DNA sequence that is occupied by TFIIIB. The 90-kDa component of TFIIIB, which only detectably interacts with the TFIIIC-DNA complex when the 70-kDa component is also present, generates the complete DNase I protection pattern of TFIIIB and bestows heparin-insensitivity on the TFIIIB-DNA complex. The resolution of TFIIIB into two functional components further defines the probable steps and interactions involved in the formation of stable transcription complexes.