A transcriptional activator is a protein (transcription factor) that increases transcription of a gene or set of genes. Activators 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 activators are DNA-binding proteins that bind to enhancers or promoter-proximal elements. The DNA site bound by the activator is referred to as an "activator-binding site". The part of the activator that makes protein–protein interactions with the general transcription machinery is referred to as an "activating region" or "activation domain".
Most activators 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 activators 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; activators also function to allow these “stalled” RNA polymerases to continue transcription.
The activity of activators can be regulated. Some activators have an allosteric site and can only function when a certain molecule binds to this site, essentially turning the activator on. Post-translational modifications to activators can also regulate activity, increasing or decreasing activity depending on the type of modification and activator being modified.
In some cells, usually eukaryotes, multiple activators can bind to the binding-site; these activators tend to bind cooperatively and interact synergistically.
Activator proteins consist of two main domains: a DNA-binding domain that binds to a DNA sequence specific to the activator, and an activation domain that functions to increase gene transcription by interacting with other molecules.
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In molecular genetics, a repressor is a DNA- or RNA-binding protein that inhibits the expression of one or more genes by binding to the operator or associated silencers. A DNA-binding repressor blocks the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes into messenger RNA. An RNA-binding repressor binds to the mRNA and prevents translation of the mRNA into protein. This blocking or reducing of expression is called repression.
Histone acetyltransferases (HATs) are enzymes that acetylate conserved lysine amino acids on histone proteins by transferring an acetyl group from acetyl-CoA to form ε-N-acetyllysine. DNA is wrapped around histones, and, by transferring an acetyl group to the histones, genes can be turned on and off. In general, histone acetylation increases gene expression. In general, histone acetylation is linked to transcriptional activation and associated with euchromatin.
A transcriptional activator is a protein (transcription factor) that increases transcription of a gene or set of genes. Activators 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 activators are DNA-binding proteins that bind to enhancers or promoter-proximal elements. The DNA site bound by the activator is referred to as an "activator-binding site".
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