Complex interactions between yeast TFIIIB and TFIIIC.

Transcription of yeast class III genes requires the sequential assembly of the general transcription factors TFIIIC and TFIIIB, and of RNA polymerase III, into an initiation complex composed of at least 25 polypeptides. The 70-kDa subunit of TFIIIB (TFIIIB70) is central in this network of interactions as it contacts both TATA-binding protein and a subunit of polymerase III. We show here that the TATA-binding protein interacts with the carboxyl-terminal part of TFIIIB70. TFIIIB70 also contacts TFIIIC (factor tau) via its tau 131 subunit. The protein domains of tau 131 and TFIIIB70 involved in this interaction, either positively or negatively, were mapped using the two-hybrid system. We provide evidence that intramolecular interactions mask functional domains in both polypeptides.