The Mediator Complex Transmits Information to RNA Polymerase

In prokaryotes, sigma factors recognize the promoter and activators generally help RNA polymerase to bind to the promoter. In eukaryotes, recognition and binding to the promoter are both functions of the general transcription factors. Activators in eukaryotes may be viewed as granting RNA polymerase permission to proceed forward from the promoter. Some eukaryotic activators make contact with the general transcription factors TFIIB, TFIID, and TFIIH. However, this is not sufficient to initiate transcription.

The mediator is a protein complex that sits on top of RNA polymerase II and provides a site of contact for activators, especially those that are bound at enhancer sequences (Fig. 10.03). The mediator consists of about 20 protein subunits and receives signals from activators. Apparently, it combines the signals from multiple activators and/or repressors and sends the final result to the RNA polymerase II enzyme. Some subunits of the mediator act in a positive manner while others act in a negative manner.

mediator A protein complex that transmits the signal from transcription factors to the RNA polymerase in eukaryotic cells

DNA GAL4 _site

Promoter

DNA LexA site

Promoter

Natural GAL4 protein

Natural GAL4 protein

GAL4 activator

LexA DNA-binding

Hybrid protein

A) Binding & transcription

A) Binding & transcription

DNA GAL4 Promoter

DNA GAL4 Promoter

No binding

DNA GAL4 Promoter

DNA LexA site

No binding

DNA LexA Promoter

No binding

DNA LexA Promoter

D) Binding & transcription

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