Figure 7.

A model for SAGA-dependent regulation of a Ste11 target gene. In this model, the SAGA coactivator complex undergoes a conformational change in response to nutrient levels at the promoters of Ste11 target genes. This conformational change controls the opposing roles for two of its subunits, Gcn5 and Spt8. (A) In rich conditions, the transcription activator Rst2, and possibly Ste11, recruits SAGA to the promoter of a Ste11 target gene, such as ste11⁺ itself. In these conditions, Rst2 is not activated and allows only basal levels of transcription that do not require SAGA. Rather, SAGA is in a conformation where Gcn5 prevents SAGA-dependent activation by inhibiting Spt8 activity. (B) Upon nutrient starvation, Rst2 and Ste11 become activated and allow the full induction of expression of the Ste11 target gene. In addition, SAGA undergoes a conformational change, thereby allowing it to function as a positive regulator, which contributes to the full induction of transcription. Under these conditions, the repressive activity of Gcn5 is inhibited in SAGA by Spt8.