DNA is packaged into nucleosomes and higher-order chromatin structures
Activators bind regulatory elements
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remodeling complexes |
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modifying complexes |
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Activators recruit chromatin remodeling and modifying complexes
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Polymerase II holoenzyme |
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TFIID
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Activators recruit components of the transcription apparatus
Activators may stimulate the activity of the assembled transcription apparatus
Several themes are featured. Transcriptional activators bind to specific DNA sequences, and the chromatin context of the DNA biding site can have a positive or negative influence on binding of the activator or the proteins it recruits to the promoter. Activators can bind and recruit chromatin remodeling and modifying complexes that influence local chromatin structure. One function of chromatin remodeling may be to increase the stability of the activator-DNA complex, and another to affect access of promoter sequences for binding of the transcription apparatus. Activators also bind and recruit the transcription initiation apparatus to promoters, probably through the concerted interactions of a few large complexes. Note that most of these processes are reversible, and are regulated by transcriptional repressors. Activators may also influence events subsequent to assembly of the initiation apparatus such as promoter clearance and RNA polymerase processivity. The diagram suggests an order to the process of gene activation, although it is not clear that this occurs at all promoters in vivo. The diagram shows a single activator bound at the promoter, but promoters typically contain multiple activator binding sites.