Kin28 regulates the transient association of Mediator with core promoters

Mediator is an essential, broadly used eukaryotic transcriptional coactivator. How and what Mediator communicates from activators to RNA polymerase II (RNAPII) remains an open question. Here we performed genome-wide location profiling of Saccharomyces cerevisiae Mediator subunits. Mediator is not found at core promoters but rather occupies the upstream activating sequence, upstream of the pre-initiation complex. In the absence of Kin28 (CDK7) kinase activity or in cells in which the RNAPII C-terminal domain is mutated to replace Ser5 with alanine, however, Mediator accumulates at core promoters together with RNAPII. We propose that Mediator is released quickly from promoters after phosphorylation of Ser5 by Kin28 (CDK7), which also allows for RNAPII to escape from the promoter.