Mediator is a multiprotein complex that functions as a transcriptional coactivator. The Mediator complex is required for the successful transcription of nearly all class II gene promoters in yeast. It works in the same manner in mammals.
The mediator functions as a coactivator and binds to the C-terminal domain (CTD) of RNA polymerase II holoenzyme, acting as a bridge between this enzyme and transcription factors.