Mediator requirement downstream of chromatin remodeling during transcriptional activation of CHA1 in yeast

Mediator complex is essential for transcription by RNA polymerase II in eukaryotes. Although chromatin remodeling is an integral part of transcriptional activation at many promoters, whether Mediator is required for this function has not been determined. Here we have used the yeast CHA1 gene to study the role of Mediator in chromatin remodeling and recruitment of the transcription machinery. We show by chromatin immunoprecipitation that Mediator subunits are recruited to the induced CHA1 promoter. Inactivation of Mediator at 37 degrees C in yeast harboring the srb4-138 (med17) ts mutation severely reduces CHA1 activation and prevents recruitment to the induced CHA1 promoter of Med18/Srb5, from the head module of Mediator, and Med14/Rgr1, which bridges the middle and tail modules. In contrast, recruitment of Med15/Gal11 from the tail module is unaffected in med17 ts yeast at 37 degrees C. Recruitment of TATA-binding protein (TBP) is severely compromised in the absence of functional Mediator, whereas Kin28 and polymerase II recruitment are reduced but to a lesser extent. Induced levels of histone H3K4me3 at the CHA1 promoter are not diminished by inactivation of Mediator, whereas recruitment of Paf1 and of Ser(2)- and Ser(5)- phosphorylated forms of Rbp1 are reduced but not eliminated. Loss of histone H3 from the induced CHA1 promoter is seen in wild type yeast but is greatly reduced by loss of intact Mediator. In contrast, Swi/Snf recruitment and nucleosome remodeling are unaffected by loss of Mediator function. Thus, Mediator is required for recruitment of the transcription machinery subsequent to chromatin remodeling during CHA1 induction.