A novel subunit of yeast RNA polymerase III interacts with the TFIIB-related domain of TFIIIB70

There is limited information on how eukaryotic RNA polymerases (Pol) recogn ize their cognate preinitiation complex. We have characterized a polypeptid e copurifying with yeast Pol III. This protein, C17, was found to be homolo gous to a mammalian protein described as a hormone receptor. Deletion of th e corresponding gene, RPC17, was lethal and its regulated extinction caused a selective defect in transcription of class III genes in vivo. Two-hybrid and coimmunoprecipitation experiments indicated that C17 interacts with tw o Pol III subunits, one of wihich, C31, is important for the initiation rea ction. C17 also interacted with TFIIIB70, the TFIIB-related component of TF IIIB, The interaction domain was found to be in the N-terminal, TFIIB-like half of TFIIIB70, downstream of the zinc ribbon and first imperfect repeat. Although Pol II similarly interacts with TFIIB, it is notable that C17 has no similarity to any Pol II subunit, The data indicate that C17 is a novel specific subunit of Pol III which participates together with C34 in the re cruitment of Pol III by the preinitiation complex.