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Originally published as Genetics Published Articles Ahead of Print on February 1, 2006.

Genetics, Vol. 172, 2169-2184, April 2006, Copyright © 2006
doi:10.1534/genetics.105.052738

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A Functional Module of Yeast Mediator That Governs the Dynamic Range of Heat-Shock Gene Expression

Harpreet Singh, Alexander M. Erkine1,2, Selena B. Kremer1, Harry M. Duttweiler, Donnie A. Davis, Jabed Iqbal, Rachel R. Gross and David S. Gross3

Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, Shreveport, Louisiana 71130-3932

3 Corresponding author: Department of Biochemistry and Molecular Biology, Louisiana State University Health Sciences Center, 1501 Kings Highway, Shreveport, LA 71130-3932.
E-mail: dgross{at}lsuhsc.edu

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 noninduced heat-shock gene expression is unknown. In an effort to identify these factors, we isolated spontaneous extragenic suppressors of hsp82-{Delta}HSE1, 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 of the null phenotype of hsp82-{Delta}HSE1 were isolated and characterized, and they sorted into six expression without heat-shock element (EWE) 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-{Delta}HSE1, 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 diminishing the dynamic range of heat-shock gene expression. Notably, targeted deletion of other Mediator subunits, including the negative regulators Cdk8/Srb10, Med5/Nut1, and Med15/Gal11 fail to derepress hsp82-{Delta}HSE1. Taken together, our data suggest that the Ewe subunits constitute a distinct functional module within Mediator that modulates both basal and induced heat-shock gene transcription.




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