A suppressor of mutations in the class III transcription system encodes a component of yeast TFIIIB

Class III genes depend on TFIIIB for recruitment of RNA polymerase III. Yeast TFIIIB is comprised of three components: TBP, TFIIIB70 and a 90 kDa polypeptide contained in the fraction B ''. We report the isolation of the yeast gene TFC7 which, based on genetic and biochemical evidence, encodes the 90 kDa polypeptide. TFC7 was isolated as a multicopy suppressor of temperature-sensitive mutations in the two largest subunits of TFIIIC. It is an essential gene, encoding a polypeptide of 68 kDa migrating with an apparent size of similar to 90 kDa. In gel shift assays, recombinant TFC7 protein (rTFC7p) alone did not bind detectably to DNA, or to the TFIIIC-DNA complex even in the presence of TBP or TFIIIB70, but it was required to assemble the TFIIIB-TFIIIC-DNA complex. The two-hybrid assay pointed to an interaction between TFC7 protein and tau(131), the second largest subunit of TFIIIC (that also interacts with TFIIIB70). rTFC7p can replace the B '' component of TFIIIB for synthesis of U6 RNA in a system reconstituted with recombinant TBP and TFIIIB70 polypeptides and highly purified RNA polymerase III. Surprisingly, specific transcription of the SUP4 tRNA(Tyr) gene promoted by rTFC7p was much weaker than with B ''. An additional factor activity, provided by the recently identified TFIIIE fraction, was required to restore control levels of transcription.