Mediator, not holoenzyme, is directly recruited to the heat shock promoter by HSF upon heat shock.

Activators of RNA polymerase II (Pol II) transcription have been shown to bind several coactivators and basal factors in vitro. Whether such interactions play a primary regulatory role in recruiting these factors to activator-associated chromosomal target sites in living cells remains unclear. Here, we show that upon heat shock the Pol II-free form of Mediator is rapidly recruited to HSF binding sites. Unlike the TAFs and Pol II, the interaction between Mediator and HSF on chromosomal loci is direct and mechanistically separable from the preinitiation complex assembly step. Therefore, the activator-Mediator interaction likely underlies the initiation of signal transfer from enhancer-bound activators to the basal transcription machinery.