Mediator-dependent recruitment of TFIIH modules in preinitiation complex

In vitro, without Mediator, the association of general transcription factors (GTF) and RNA polymerase II (Pol II) in preinitiation complexes (PIC) occurs in an orderly fashion. In this work, we explore the in vivo function of Mediator in GTF recruitment to PIC. A direct interaction between Med11 Mediator head subunit and Rad3 TFIIH subunit was identified. We explored the significance of this interaction and those of Med11 with head module subunits Med17 and Med22 and found that impairing these interactions could differentially affect the recruitment of TFIIH, TFIIE, and Pol II in the PIC. A med11 mutation that altered promoter occupancy by the TFIIK kinase module of TFIIH genome-wide also reduced Pol II CTD serine 5 phosphorylation. We conclude that the Mediator head module plays a critical role in TFIIH and TFIIE recruitment to the PIC. We identify steps in PIC formation that suggest a branched assembly pathway.