The Mediator complex

What is it? The Mediator complex is an enormous, multi-subunit protein complex that regulates gene expression in eukaryotic cells. It acts as a molecular circuit board that integrates physiological and developmental signals and transmits them to RNA polymerase II (Pol II), the enzyme charged with transcription of all protein-encoding genes. Although initially discovered as a cofactor required for activatordependent transcription, Mediator also stimulates activator-independent (basal) transcription. Furthermore, Mediator can act as a co-repressor that facilitates transcriptional silencing. Mediator activity is not limited to its effects on transcription initiation, but it can also affect elongation and perhaps all stages of the transcription cycle.