repressor

Meaning: A repressor is a DNA-binding regulatory protein that lowers transcription of a gene. In bacteria it binds the operator, a sequence near the promoter, and blocks RNA polymerase from binding or moving forward. Its shape can be changed by a small molecule. An inducer (allolactose in the lac operon) binds the repressor and releases it from DNA, so transcription rises. A corepressor (tryptophan in the trp operon) binds an inactive repressor and lets it grip DNA, so transcription falls. In eukaryotes, repressors often bind silencers or interfere with activators.

On the MCAT: Passages give an operon plus a mutation or a changed condition and ask whether the genes are expressed. A repressor that cannot bind DNA, or an operator the repressor cannot recognize, gives constitutive expression. The lac operon is inducible (normally off); the trp operon is repressible (normally on). With lactose present but glucose high, lac expression stays low because the CAP–cAMP activator is missing, so the repressor is not the only control.

Common confusion: A repressor acts on transcription, not translation, and it is not permanently bound to DNA. Whether it sits on the operator depends on the small molecule: an inducer removes it, a corepressor helps it bind. The repressor is also not the operator, which is the DNA site it binds.