Mediator complex

Picture a translator standing between two people who do not speak the same language: the activators perched on a distant enhancer, and the giant RNA polymerase machine waiting at the promoter. The activators want the polymerase to start, but they cannot speak to it directly. The Mediator complex is that translator — a large assembly of proteins that relays the 'start' message from the regulators to the core machinery.

The Mediator is a complex of two to three dozen protein subunits that sits between gene-specific transcription factors and RNA polymerase II. One end of it grips the activation domains of transcription factors bound at enhancers and promoters; the other end docks onto RNA polymerase II and its general transcription factors at the promoter. By bridging the two, Mediator transmits the regulatory signal: when the right activators are present and contacting Mediator, it helps assemble the pre-initiation complex, stimulates the polymerase, and is needed to position it for a productive start. Because an enhancer is often far from a promoter, Mediator works hand in hand with DNA looping and with cohesin to bring the two ends together physically. It is a hub, not a switch — it integrates many incoming contacts into a single output at the polymerase.

Mediator matters because almost all regulated transcription by RNA polymerase II flows through it; it is the common conduit where the thousands of different transcription factors converge on one machine. That central role makes it a sensitive control point and a vulnerability: mutations in particular Mediator subunits cause developmental disorders and are recurrently found in cancers, precisely because changing the translator garbles many messages at once. A useful caution: Mediator is a coactivator, meaning it does not bind specific DNA sequences itself — it is recruited by the factors that do, so it has no opinion about which gene to turn on, only about relaying the decision once it has been made.