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Molecular and Cellular Biology, August 2004, p. 6871-6886, Vol. 24, No. 15
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.15.6871-6886.2004

A Triad of Subunits from the Gal11/Tail Domain of Srb Mediator Is an In Vivo Target of Transcriptional Activator Gcn4p

Fan Zhang,{dagger} Laarni Sumibcay, Alan G. Hinnebusch,* and Mark J. Swanson{dagger},{ddagger}

Laboratory of Gene Regulation and Development, National Institute of Child Health and Human Development, Bethesda, Maryland 20892

Received 15 March 2004/ Returned for modification 18 April 2004/ Accepted 9 May 2004

The Srb mediator is an important transcriptional coactivator for Gcn4p in the yeast Saccharomyces cerevisiae. We show that three subunits of the Gal11/tail domain of mediator, Gal11p, Pgd1p, and Med2p, and the head domain subunit Srb2p make overlapping contributions to the interaction of mediator with recombinant Gcn4p in vitro. Each of these proteins, along with the tail subunit Sin4p, also contributes to the recruitment of mediator by Gcn4p to target promoters in vivo. We found that Gal11p, Med2p, and Pgd1p reside in a stable subcomplex in sin4{Delta} cells that interacts with Gcn4p in vitro and that is recruited independently of the rest of mediator by Gcn4p in vivo. Thus, the Gal11p/Med2p/Pgd1p triad is both necessary for recruitment of intact mediator and appears to be sufficient for recruitment by Gcn4p as a free subcomplex. The med2{Delta} mutation impairs the recruitment of TATA binding protein (TBP) and RNA polymerase II to the promoter and the induction of transcription at ARG1, demonstrating the importance of the tail domain for activation by Gcn4p in vivo. Even though the Gal11p/Med2p/Pgd1p triad is the only portion of Srb mediator recruited efficiently to the promoter in the sin4{Delta} strain, this mutant shows high-level TBP recruitment and wild-type transcriptional induction at ARG1. Hence, the Gal11p/Med2p/Pgd1p triad may contribute to TBP recruitment independently of the rest of mediator.

* Corresponding author. Mailing address: NIH, Building 6A/Room B1A13, Bethesda, MD 20892. Phone: (301) 496-4480. Fax: (301) 496-6828. E-mail: ahinnebusch{at}nih.gov.

{dagger} F.Z. and M.J.S. contributed equally.

{ddagger} Present address: School of Biological Sciences, Louisiana Tech University, Ruston, LA 71272-0001.

Molecular and Cellular Biology, August 2004, p. 6871-6886, Vol. 24, No. 15
0022-538X/04/$08.00+0     DOI: 10.1128/MCB.24.15.6871-6886.2004




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