How do operons regulate gene expression?

Repressible operons are switched on unless their repressor is activated. They have a promoter, where RNA polymerase binds. However, if enough of the molecule created by this gene is present, the molecule acts as a corepressor and activates the repressor. The activated repressor binds to the operator, preventing RNA polymerase from binding to the promoter. This way, having produced enough of the molecule causes the production of this molecule to be stopped.

Inducible operons are switched off by default, using an active repressor. In order for the gene to be transcribed, the repressor must be prevented from binding to the operator, allowing RNA polymerase to bind to the promoter region. The repressor is inactivated by an inducer molecule that binds to the repressor.