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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