What happens when a corepressor binds to the repressor in a repressible operon?

• A. Induces RNA processing changes.
• B. RNA polymerase binds more strongly.
• C. Increases transcription by opening chromatin.
• D. Repressor becomes active, blocking transcription.

Answer: (D)

When a repressor binds a corepressor in a repressible operon, the corepressor activates the repressor. The activated repressor then binds to the operator and blocks RNA polymerase from initiating or continuing transcription, turning off gene expression. This setup is a negative feedback control: the end product (the corepressor) signals that enough of the product is present and stops production. The other options don’t fit this bacterial mechanism—RNA processing changes don’t occur in prokaryotes, stronger RNA polymerase binding would increase transcription, and opening chromatin isn’t a feature of bacterial regulation.