ON THE SUBUNIT COMPOSITION, STOICHIOMETRY, AND PHOSPHORYLATION OF THEYEAST TRANSCRIPTION FACTOR-TFIIIC TAU/

Saccharomyces cerevisiae transcription factor IIIC/tau is a multisubun it DNA-binding protein that plays key roles in tRNA and 5 S rRNA gene activation. Subunit composition, stoichiometry, and in vivo phosphoryl ation of TFIIIC/tau factor were investigated using factor prepared fro m strains carrying modified forms of TFC1, the gene encoding the 95-kD a TFIIIC/tau subunit (tau95). Using an epitope-tagged TFC1 as well as a TFC1-lacZ fusion, TFIIIC was shown to contain a single 95-kDa subuni t, which was localized by electron microscopy into tauA, the A block-b inding domain of TFIIIC/tau. Three S-35-labeled polypeptides (at 138, 131, and 91 kDa) coimmunoprecipitated with a tau95-beta-galactosidase fusion protein. The coprecipitation of the 91-kDa polypeptide makes it a likely subunit of the factor. Immunoprecipitation from P-32-labeled extracts revealed that three of the subunits (138, 131, and 95 kDa), but not the 91-kDa component, are phosphorylated in vivo.