Role of the yeast pre-initiation complex in Mediator recruitment and dynamics

The Mediator complex plays a crucial role in eukaryotic transcription by RNA Polymerase II (Pol II) by stimulating the cooperative assembly of a pre-initiation complex (PIC) and recruitment of Pol II for gene activation. In yeast, Mediator recruitment has generally been ascribed to sequence-specific activators engaging the tail module triad of Med2-Med3-Med15 at the UAS. We show that med2∆ med3∆ med15∆ yeast are viable and that Mediator lacking Med2-Med3-Med15 is associated with active promoters genome-wide. To test for PIC dependence of Mediator association, we conducted ChIP-seq against Mediator subunits after depleting components of the PIC. Mediator occupancy at proximal promoters is diminished after depletion of Kin28 together with TBP, Pol II, or Taf1, particularly at TATA-less promoters, indicating a role for the PIC in stabilizing Mediator association. We also show that TBP plays a critical role in the ability of Mediator to bridge upstream activating sequences with proximal promoter regions.