repressor

A repressor is the “off” guard at a gene. It is a protein that sits on a specific stretch of DNA and physically gets in the way, so the transcription machinery cannot run the gene. As long as the repressor is bound, the gene stays quiet.

In bacteria, repressors often bind a site called the operator, blocking RNA polymerase or stopping it from moving forward. The binding is usually reversible and responsive: a small molecule can latch onto the repressor and change its shape so it lets go of the DNA, switching the gene on; in other systems a small molecule does the opposite and helps the repressor clamp down. This is the essence of negative control — a gene held off until a signal releases the brake.

Repressors are not unique to bacteria. Eukaryotes also use repressor proteins, often working through chromatin and assembled complexes rather than a single operator. Either way, the logic is the same: regulate by inhibiting, blocking expression until conditions call for the gene.