Specific components of the SAGA complex are required for Gcn4- and Gcr1-mediated activation of the his4-912 delta promoter in Saccharomyces cerevisiae

Mutations selected as suppressors of Ty or solo delta insertion mutations i n Saccharomyces cerevisiae have identified several genes, SPT3, SPT7, SPT8, and SPT20, that encode components of the SAGA complex. However, the mechan ism by which SAGA activates transcription of specific RNA polymerase II-dep endent genes is unknown. We have conducted a fine-structure mutagenesis of one widely used SAGA-dependent promoter, the delta element of his4-912 delt a, to identify sequence elements important for its promoter activity. Our a nalysis has characterized three delta regions necessary for full promoter a ctivity and accurate start site selection: an upstream activating sequence, a TATA region, and an initiator region. In addition, we have shown that fa ctors present at the adjacent UAS(HIS4) (Gcn4, Bas1, and Pho2) also activat e the delta promoter in his4-912 delta. Our results suggest a model in whic h the delta promoter in his4-912 delta is primarily activated by two factor s: Gcr1 acting at the UAS(delta) and Gcn4 acting at the UAS(HIS4). Finally, we tested whether activation by either of these factors is dependent on co mponents of the SAGA complex. Our results demonstrate that Spt3 and Spt20 a re required for full delta promoter activity, but that Gcn5, another member of SAGA, is not required. Spt3 appears to be partially required for activa tion of his4-912 delta by both Gcr1 and Gcn4. Thus, our work suggests that SAGA exerts a large effect on delta promoter activity through a combination of smaller effects on multiple factors.