Structure of the tau60/Delta tau91 subcomplex of yeast transcription factor IIIC: insights into preinitiation complex assembly.

Yeast RNA polymerase III is recruited upon binding of subcomplexes tauA and tauB of transcription factor IIIC (TFIIIC) to the A and B blocks of tRNA gene promoters. The tauB subcomplex consists of subunits tau60, tau91, and tau138. We determined the 3.2 A crystal structure of tau60 bound to a large C-terminal fragment of tau91 (Deltatau91). Deltatau91 protein contains a seven-bladed propeller preceded by an N-terminal extension, whereas tau60 contains a structurally homologous propeller followed by a C-terminal domain with a novel alpha/beta fold. The two propeller domains do not have any detectable DNA binding activity and mediate heterodimer formation that may serve as scaffold for tau138 assembly. We show that the C-terminal tau60 domain interacts with the TATA binding protein (TBP). Recombinant tauB recruits TBP and stimulates TFIIIB-directed transcription on a TATA box containing tRNA gene, implying a combined contribution of tauA and tauB to preinitiation complex formation.