activator

An activator is the “go” signal for a gene. It is a protein that binds DNA near a gene and makes transcription happen more readily — pressing the accelerator rather than releasing a brake. Where a repressor blocks, an activator promotes.

Typically an activator binds a regulatory site and then helps recruit RNA polymerase or stabilise it at the promoter, sometimes by contacting the polymerase directly, sometimes by remodelling DNA packaging so the gene becomes accessible. In bacteria a well-known activator boosts certain operons when energy is low; in eukaryotes activators often bind enhancers and reach the promoter through DNA looping.

Activators give cells positive control: instead of merely lifting an inhibition, they actively turn genes up, and the strength and combination of activators present help set exactly how much a gene is expressed. Many activators are switched on only by a signal — a hormone, a nutrient, a stress — so they let a gene respond to the cell's circumstances.