Different upstream transcriptional activators have distinct coactivator requirements

Activated transcription by RNA polymerase II (Pol II) requires coactivators , one of which is the SRB/mediator. Whereas Srb4, an essential subunit of t he SRB/mediator, is broadly required for Pol II transcription in yeast, we have shown that it is dispensable for the transcriptional activation of som e genes. Here, we show that transcriptional activation by different natural activators, and by artificial recruitment of various transcription factors , have very different degrees of Srb4 independence. These data, and the ana lysis of an rgr1 mutant, point to an Rgr1 subcomplex of the SRB/mediator as the mechanistic route of activation by Srb4-independent activators in vivo .