Multiple roles of the τ131 subunit of yeast transcription factor IIIC (TFIIIC) in TFIIIB assembly

Yeast transcription factor IIIC (TFIIIC) plays a key role in assembling the transcription initiation factor TFIIIB on class III genes after TFIIIC-DNA binding. The second largest subunit of TFIIIC, tau 131, is thought to initiate TFIIIB assembly by interacting with Brf1/TFIIIB70. In this work, we have analyzed a TFIIIC mutant (tau 131-Delta TPR2) harboring a deletion in tau 131 removing the second of its It tetratricopeptide repeats. Remarkably, this thermosensitive mutation was selectively suppressed in vivo by overexpression of B"/TFIIIB90, but not Brf1 or TATA-binding protein. In vitro, the mutant factor preincubated at restrictive temperature bound DNA efficiently but lost transcription factor activity. The in vitro transcription defect was abolished at high concentrations of B" but not Brf1. Copurification experiments of baculovirus-expressed proteins confirmed a direct physical interaction between tau 131 and B". tau 131, therefore, appears to be involved in the recruitment of both Brf1 and B".