THE TFIIIB-ASSEMBLING SUBUNIT OF YEAST TRANSCRIPTION FACTOR TFIIIC HAS BOTH TETRATRICOPEPTIDE REPEATS AND BASIC HELIX-LOOP-HELIX MOTIFS

The multisubunit yeast transcription factor IIIC (TFIIIC; also called tau) Can undergo considerable conformational changes upon binding to t he A and B blocks of tRNA genes. After binding to DNA encoding tRNA (t DNA), TFIIIC acts as an assembly factor to recruit an initiation facto r, TFIIIB, via its tau131 subunit. We have cloned the gene encoding th e tau131 subunit and named it TFC4. This gene is unique, essential for cell viability, and encodes a M(r) 120,153 protein. Epitope-tagging a nd mobility-shift assays indicated the presence of a single tau131 sub unit in TFIIIC-tDNA complexes. Tau131 contains two sequence motifs, ac counting for nearly one-half of the protein mass, that may provide a m olecular explanation for the properties of TFIIIC-tDNA complex. A seri es of 11 copies of the tetratricopeptide repeat motif may account. for the flexibility and interaction properties of TFIIIC. A motif akin to the basic helix-loop-helix motif of MyoD suggests the direct involvem ent of tau131 in promoting DNA binding of TFIIIB.