Mediator-RNA Polymerase II Interactions Critical for Transcriptional Activation Are Mediated by the N-terminal Half of MED14 and C-terminal Domain of RPB1

Mediator is a large and evolutionarily conserved coactivator complex essential for RNA polymerase II (Pol II)-mediated gene regulation at multiple steps of the transcription process, including preinitiation complex (PIC) assembly and function. Here, we used the MultiBac baculovirus expression system to generate recombinant human core Mediator subcomplexes and subsequent biochemical approaches to dissect the mechanism by which Mediator facilitates direct recruitment of Pol II to core promoters. Our results highlight a pivotal role in this process for the N-terminal half (NTD) of the MED14 subunit. We show that a reconstituted 15-subunit human core Mediator complex that contains only the MED14-NTD is fully functional in facilitating both basal and activated (p53) transcription. This complex directly interacts with the C-terminal domain (CTD) of the RPB1 subunit of Pol II (RPB1 CTD) and is required for recruiting Pol II to core promoters. Moreover, recombinant RPB1 can completely reverse the human core Mediator-Pol II interaction. Notably, the human MED14-NTD region has secondary structure conservation with Schizosaccharomyces pombe. In addition, reanalysis of published cryo-EM structures of yeast Mediator-Pol II complexes strongly supports our conclusion. Thus, our analyses provide critical new insights into how Mediator binds to Pol II and recruits it to the promoters to facilitate transcription.