The Swi5 activator recruits the Mediator complex to theHOpromoter without RNA polymerase II

Abstract

Regulation ofHOgene expression in the yeastSaccharomyces cerevisiaeis intricately orchestrated by an assortment of gene-specific DNA-binding and non-DNA binding regulators. Binding of the early G₁transcription factor Swi5 to the distal URS1 element of theHOpromoter initiates a cascade of events through recruitment of the Swi/Snf and SAGA complexes. In late G₁, binding of transcription factor SBF to promoter proximal sequences results in the timely expression ofHO. In this work we describe an important additional layer of complexity to the current model by identifying a connection between Swi5 and the Mediator/RNA polymerase II holoenzyme complex. We show that Swi5 recruits Mediator toHOby specific interaction with the Gal11 module of the Mediator complex. Importantly, binding of both the Gal11 and Srb4 mediator components to the upstream region ofHOis independent of the SBF factor. Swi/Snf is required for Mediator binding, and genetic suppression experiments suggest that Swi/Snf and Mediator act in the same genetic pathway ofHOactivation. Experiments examining the kinetics of binding show that Mediator binds toHOpromoter elements 1.5 kb upstream of the transcription start site in early G₁, but this binding occurs without RNA Pol II. RNA Pol II does not bind toHOuntil late G₁, whenHOis actively transcribed, and binding occurs exclusively to the TATA region.