Wild-type transcriptional activation by Gcn4p is dependent on multiple coactivators, including SAGA, SWI/SNF, Srb mediator, CCR4-NOT, and RSC, which are all recruited by Gcn4p to its target promoters in vivo. It was not known whether these coactivators are required for assembly of the preinitiation complex (PIC) or for subsequent steps in the initiation or elongation phase of transcription. We find that mutations in subunits of these coactivators reduce the recruitment of TATA binding protein (TBP) and RNA polymerase II (Pol II) by Gcn4p at ARG1, ARG4, and SNZ1, implicating all five coactivators in PIC assembly at Gcn4p target genes. Recruitment of Pol II at SNZ1 and ARG1 was eliminated by mutations in TBP or by deletion of the TATA box, indicating that TBP binding is a prerequisite for Pol II recruitment by Gcn4p. However, several mutations in SAGA subunits and deletion of SRB10 had a greater impact on promoter occupancy of Pol II versus TBP, suggesting that SAGA and Srb mediator can promote Pol II binding independently of their stimulatory effects on TBP recruitment. Our results reveal an unexpected complexity in the cofactor requirements for the enhancement of PIC assembly by a single activator protein.

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Journal Article

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Qiu H, Hu C, Yoon S, Natarajan K, Swanson MJ, Hinnebusch AG

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