Abstract: We report the results of a genetic screen designed to identify transcriptional coregulators of yeast heat shock factor (HSF). This sequence-specific activator is required to stimulate both basal and induced transcription; however, the identity of factors that collaborate with HSF in governing non-induced heat shock gene expression is unknown. In an effort to identify these factors, we isolated spontaneous extragenic suppressors of hsp82-DeltaHSE1, an allele of HSP82 that bears a 32 bp deletion of its high-affinity HSF binding site, yet retains its two low-affinity HSF sites. Nearly 200 suppressors were isolated and characterized, and they sorted into six EWE (for Expression Without heat shock Element) complementation groups. Strikingly, all six groups contain alleles of genes that encode subunits of Mediator. Three of the six subunits, Med7, Med10/Nut2 and Med21/Srb7 map to Mediator's middle domain; two subunits, Med14/Rgr1 and Med16/Sin4, to its tail domain; and one subunit, Med19/Rox3, to its head domain. Mutations in genes encoding these factors enhance not only the basal transcription of hsp82- poundGHSE1, but also that of wild-type heat shock genes. In contrast to their effect on basal transcription, the more severe ewe mutations strongly reduce activated transcription, drastically reducing the dynamic range of heat shock gene expression. Notably, targeted deletion of other Mediator subunits, including the head domain component Med20/Srb2 and the negative regulators Cdk8/Srb10, Med5/Nut1, and Med15/Gal11 fail to derepress hsp82DeltaHSE1. Taken together, our data suggest that the Ewe subunits comprise a distinct functional module within Mediator that modulates both basal and induced heat shock gene transcription.