SAGA is an essential in vivo target of the yeast acidic activator Gal4p

Despite major advances in characterizing the eukaryotic transcriptional mac hinery, the function of promoter-specific transcriptional activators (activ ators) is still not understood. For example, in no case have the direct in vivo targets of a transcriptional activator been unambiguously identified, nor has it been resolved whether activators have a single essential target or multiple redundant targets. Here we address these issues for the prototy pe acidic activator yeast Gal4p. Gal4p binds to the upstream activating seq uence (UAS) of GAL1 and several other GAL genes and stimulates transcriptio n in the presence of galactose. Previous studies have shown that GAL1 trans cription is dependent on the yeast SAGA (Spt/Ada/GCN5/acetyltransferase) co mplex. Using formaldehyde-based in vivo cross-linking, we show that the Gal 4p activation domain recruits SAGA to the GAL1 UAS. If SAGA is not recruite d to the UAS, the preinitiation complex (PIC) fails to assemble at the GAL1 core promoter, and transcription does not occur. SAGA, but not other trans cription components, is also recruited by the Gal4p activation domain to a plasmid containing minimal Gal4p-binding sites. Recruitment of SAGA by Gal4 p and stimulation of PIC assembly is dependent on several SAGA subunits but not the SAGA histone acetyl-transferase (HAT) GCN5. Based on these and oth er results, we conclude that SAGA is an essential target of Gal4p that, fol lowing recruitment to the UAS, facilitates PIC assembly and transcription.