activator

A transcriptional ac†ïvâ†ør is a protein (transcription factor) that increases transcription of a gene or set of genes. Ac†ïvâ†ørs are considered to have positive control over gene expression, as they function to promote gene transcription and, in some cases, are required for the transcription of genes to occur. Most ac†ïvâ†ørs are DNA-binding proteins that bind to enhancers or promoter-proximal elements. The DNA site bound by the ac†ïvâ†ør is referred to as an "ac†ïvâ†ør-binding site". The part of the ac†ïvâ†ør that makes protein–protein interactions with the general transcription machinery is referred to as an "activating region" or "activation domain".Most ac†ïvâ†ørs function by binding sequence-specifically to a regulatory DNA site located near a promoter and making protein–protein interactions with the general transcription machinery (RNA polymerase and general transcription factors), thereby facilitating the binding of the general transcription machinery to the promoter. Other ac†ïvâ†ørs help promote gene transcription by triggering RNA polymerase to release from the promoter and proceed along the DNA. At times, RNA polymerase can pause shortly after leaving the promoter; ac†ïvâ†ørs also function to allow these “stalled” RNA polymerases to continue transcription.The activity of ac†ïvâ†ørs can be regulated. Some ac†ïvâ†ørs have an allosteric site and can only function when a certain molecule binds to this site, essentially turning the ac†ïvâ†ør on. Post-translational modifications to ac†ïvâ†ørs can also regulate activity, increasing or decreasing activity depending on the type of modification and ac†ïvâ†ør being modified.In some cells, usually eukaryotes, multiple ac†ïvâ†ørs can bind to the binding-site; these ac†ïvâ†ørs tend to bind cooperatively and interact synergistically.

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