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J. Biol. Chem., Vol. 282, Issue 8, 5551-5559, February 23, 2007

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Med19(Rox3) Regulates Intermodule Interactions in the Saccharomyces cerevisiae Mediator Complex^*

From the Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03755

The Saccharomyces cerevisiae Mediator is a 25-subunit complex that facilitates both transcriptional activation and repression. Structural and functional studies have divided Mediator subunits into four distinct modules. The Head, Middle, and Tail modules form the core functional Mediator complex, whereas a fourth, the Cyc-C module, is variably associated with the core. By purifying Mediator from a strain lacking the Med19(Rox3) subunit, we have found that a complex missing only the Med19(Rox3) subunit can be isolated under mild conditions. Additionally, we have established that the entire Middle module is released when the med19(rox3) Mediator is purified under more stringent conditions. In contrast to most models of the modular structure of Mediator, we show that release of the Middle module in the med19(rox3) Mediator leaves a stable complex made up solely of Head and Tail subunits. Both the intact and Head-Tail med19(rox3) Mediator complexes have defects in enhanced basal transcription, enhanced TFIIH phosphorylation of the CTD, as well as binding of RNA Pol II and the CTD. The largely intact med19(rox3) complex facilitates activated transcription at levels similar to the wild type Mediator. In the absence of the Middle module, however, the med19(rox3) Mediator is unable to facilitate activated transcription. Although the Middle module is unnecessary for holding the Head and Tail modules together, it is required for the complex to function as a conduit between activators and the core transcription machinery.

Received for publication, October 6, 2006 , and in revised form, December 21, 2006.

^* This work was supported by National Institutes of Health Grant GM62483 (to L. C. M.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed. Tel.: 603-650-1198; Fax: 603-650-1128; E-mail: larry.myers{at}dartmouth.edu.

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