Structure of the τ60/Δτ91 Subcomplex of Yeast Transcription Factor IIIC: Insights into Preinitiation Complex Assembly

SummaryYeast RNA polymerase III is recruited upon binding of subcomplexes τA and τB of transcription factor IIIC (TFIIIC) to the A and B blocks of tRNA gene promoters. The τB subcomplex consists of subunits τ60, τ91, and τ138. We determined the 3.2 Å crystal structure of τ60 bound to a large C-terminal fragment of τ91 (Δτ91). Δτ91 protein contains a seven-bladed propeller preceded by an N-terminal extension, whereas τ60 contains a structurally homologous propeller followed by a C-terminal domain with a novel α/β fold. The two propeller domains do not have any detectable DNA binding activity and mediate heterodimer formation that may serve as scaffold for τ138 assembly. We show that the C-terminal τ60 domain interacts with the TATA binding protein (TBP). Recombinant τB recruits TBP and stimulates TFIIIB-directed transcription on a TATA box containing tRNA gene, implying a combined contribution of τA and τB to preinitiation complex formation.