Transcriptional activation by Gcn4p involves independent interactions withthe SWI/SNF complex and the SRB/mediator

Mutations in three subunits of the SWI/SNF complex and in the Med2p subunit of the SRB/mediator of pol II holoenzyme impaired Gcn4p-activated transcri ption of HIS3 without reducing Gcn4p-independent transcription of this gene . Recombinant Gcn4p interacted with SWI/SNF and SRB/mediator subunits in ce ll extracts in a manner dependent on the same hydrophobic clusters in the G cn4p activation domain; however, higher concentrations of Gcn4p were requir ed for binding to SWI/SNF versus SRB/mediator subunits. In addition, SRB/me diator and SWI/SNF subunits did not coimmunopreciptate from the extracts. T hese findings, together with the fact that Gcn4p specifically interacted wi th purified SWI/SNF, strongly suggest that Gcn4p independently recruits SWI /SNF and holoenzyme to its target promoters in the course of activating tra nscription.