Independent recruitment in vivo by Gal4 of two complexes required for transcription.

We use a modified form of ChIP to analyze the recruitment of seven sets of proteins to the yeast GAL genes upon induction. We resolve three stages of recruitment: first SAGA, then Mediator, and finally Pol II along with four other proteins (including TBP) bind the promoter. In a strain lacking SAGA, Mediator is recruited with a time course indistinguishable from that observed in wild-type cells. Our results are consistent with the notion that a single species of activator, Gal4, separately contacts, and thereby directly recruits, SAGA and Mediator.