Specific Components of the SAGA Complex Are Required for Gcn4- and Gcr1-Mediated Activation of the his4-912δ Promoter in Saccharomyces cerevisiae

Abstract

Abstract Mutations selected as suppressors of Ty or solo δ insertion mutations in Saccharomyces cerevisiae have identified several genes, SPT3, SPT7, SPT8, and SPT20, that encode components of the SAGA complex. However, the mechanism by which SAGA activates transcription of specific RNA polymerase II-dependent genes is unknown. We have conducted a fine-structure mutagenesis of one widely used SAGA-dependent promoter, the δ element of his4-912δ, to identify sequence elements important for its promoter activity. Our analysis has characterized three δ regions necessary for full promoter activity and accurate start site selection: an upstream activating sequence, a TATA region, and an initiator region. In addition, we have shown that factors present at the adjacent UASHIS4 (Gcn4, Bas1, and Pho2) also activate the δ promoter in his4-912δ. Our results suggest a model in which the δ promoter in his4-912δ is primarily activated by two factors: Gcr1 acting at the UASδ and Gcn4 acting at the UASHIS4. Finally, we tested whether activation by either of these factors is dependent on components of the SAGA complex. Our results demonstrate that Spt3 and Spt20 are required for full δ promoter activity, but that Gcn5, another member of SAGA, is not required. Spt3 appears to be partially required for activation of his4-912δ by both Gcr1 and Gcn4. Thus, our work suggests that SAGA exerts a large effect on δ promoter activity through a combination of smaller effects on multiple factors.