Mediator of transcriptional regulation.

A multi-protein complex, termed mediator, has been isolated from yeast, based on its requirement for transcriptional activation in a system reconstituted from pure RNA polymerase II and general transcription factors. Mediator polypeptides include the products of many genes previously recovered from screens for mutations affecting transcription. This connection between biochemical and genetic studies reveals that mediator is important for both activation and repression of transcription, and that mediator plays a role in transcriptional regulation in vivo as well as in vitro. Mediator binds the carboxy-terminal domain of RNA polymerase II, forming a polymerase holoenzyme, whose possible association with additional proteins is a subject of some controversy.