Endogenous mediators are intracellular proteins that enhance and activate the functions of other proteins. This term is commonly used in molecular biology and cell biology to describe a specific protein complex that acts in transcription as a coactivator. This Mediator complex is located within the cell nucleus.

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 of RNA polymerase II holoenzyme, acting as a bridge between this enzyme and transcription factors.

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