τ91, an essential subunit of yeast transcription factor IIIC, cooperates with τ138 in DNA binding

Transcription factor IIIC (TFIIIC) (or tau) is a large multisubunit and multifunctional factor required for transcription of all class III genes in Saccharomyces cerevisiae. It is responsible for promoter recognition and TFIIIB assembly. We report here the cloning and characterization of TFC6, an essential gene encoding the 91-kDa polypeptide, tau 91, present in affinity-purified TFIIIC. tau 91 has a predicted molecular mass of 74 kDa. It harbors a central cluster of His and Cys residues and has basic and acidic amino acid regions, but it shows no specific similarity to known proteins or predicted open reading frames. The TFIIIC subunit status of tau 91 was established by the following biochemical and genetic evidence. Antibodies to tau 91 bound TFIIIC-DNA complexes in gel shift assays; in vivo, a B block deficient U6 RNA gene (SNR6) harboring GALA binding sites was reactivated by fusing the GAL4 DNA binding domain to tau 91; and a point mutation in TFC6 (tau 91-E330K) was found to suppress the thermosensitive phenotype of a tfc3-G349E mutant affected in the B block binding subunit (tau 138). The suppressor mutation alleviated the DNA binding and transcription defects of mutant TFIIIC in vitro. These results indicated that tau 91 cooperates with tau 138 for DNA binding. Recombinant tau 91 by itself did not interact with a tRNA gene, although it showed a strong affinity for single-stranded DNA.