A coactivator is a protein increases gene expression by binding to an activator which contains a DNA binding domain. The coactivator is unable to bind DNA by itself.
The coactivator increases the rate of initiation by stabilising the formation of the RNA polymerase holoenzyme enabling faster clearance of the promoter
The same coactivator will likely be used to increase transcription of many different genes, since it is the actiator the provides the specificity to a particular sequence.
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