operator

An operator is the keyhole that a repressor's key fits into. It is a short DNA sequence, sitting at or near a gene's promoter in bacteria, where a repressor protein can bind. When the repressor is on the operator, the gene downstream is held off.

Because the operator overlaps or neighbours the promoter, a repressor parked there gets in the way of RNA polymerase, preventing transcription of the operon's genes. Remove the repressor — for instance when a signal molecule changes its shape — and the operator is free, so polymerase can proceed. The operator is thus the physical address on the DNA where negative control is exercised.

Operators are a defining feature of the operon model in bacteria. They illustrate a key idea in gene regulation: control often comes down to whether a particular protein is or is not sitting on a specific short patch of DNA, and small molecules in the cell tip that balance one way or the other.