Persistent Mediator association with genes repressed by heat shock or CdCl2 exposure in budding yeast

Mediator occupies proximal promoter regions only transiently prior to Pol II escape; interruption of the transcription cycle by inactivation or depletion of Kin28 inhibits Pol II escape and stabilizes Mediator occupancy at promoters. However, whether Mediator occupancy and dynamics differ for gene cohorts induced by stress or alternative growth conditions has not been examined on a genome-wide scale. Here we investigate Mediator occupancy following heat shock or CdCl2 induction, with or without depletion of Kin28. We find that Pol II occupancy exhibits similar dependence on Mediator under normal and heat shock conditions, and that Mediator occupancy at promoters increases upon Kin28 depletion after heat shock as it does under normal growth conditions, indicating similar transient occupancy. Unexpectedly, Mediator occupancy persists at genes repressed by heat shock or CdCl2 induction, and is shifted upstream from the proximal promoter region under conditions of Kin28 depletion, suggesting that Mediator is recruited by activators but is unable to engage PIC components at these repressed targets. Finally, we show a reduced dependence on PIC components for Mediator occupancy at promoters after heat shock, suggesting an altered dynamics or stronger engagement with activators under these conditions.