Activator-specific requirement of yeast mediator proteins for RNA polymerase II transcriptional activation

The multisubunit Mediator complex of Saccharomyces cerevisiae is required f or most RNA polymerase II (Pol II) transcription. The Mediator complex is c omposed of two subcomplexes, the Rgr1 and Srb4 subcomplexes, which appear t o function in the reception of activator signals and the subsequent modulat ion of Pol II activity, respectively. In order to determine the precise com position of the Mediator complex and to explore the specific role of each M ediator protein, our goal was to identify all of the Mediator components. T o this end, we cloned three previously unidentified Mediator subunits, Med9 /Cse2, Med10/Nut2, and Med11, and isolated mutant forms of each of them to analyze their transcriptional defects. Differential display and Northern an alyses of mRNAs from wild-type and Mediator mutant cells demonstrated an ac tivator-specific requirement for each Mediator subunit. Med9/Cse2 and Med10 /Nut2 were required, respectively, for Bas1/Bas2- and Gcn4-mediated transcr iption of amino acid biosynthetic genes. Gal11 was required for Gal4- and R ap1-mediated transcriptional activation. Med11 was also required specifical ly for MF alpha 1 transcription. On the other hand, Med6 was required for a ll of these transcriptional activation processes. These results suggest tha t distinct Mediator proteins in the Rgr1 subcomplex are required for activa tor-specific transcriptional activation and that the activation signals med iated by these Mediator proteins converge on Med6 (or the Srb4 subcomplex) to modulate Pol II activity.