A _____ operon is usually active, unless turned off somehow.

A _____ operon is usually active, unless turned off somehow.

• A. Inducible
• B. Constitutive
• C. Lac operon
• D. Repressible

Answer: (D)

Regulation where the default state is transcriptionally on and can be switched off when enough end product is present is repressible control. In repressible operons, the pathway’s end product serves as a corepressor that binds to a repressor protein, enabling it to attach to the operator and block RNA polymerase, turning off transcription. This means the operon remains active while product levels are low, but shuts down once there’s plenty of product to conserve resources. The lac operon, by contrast, is an inducible system that is usually off and turned on by an inducer; constitutive expression would be continuously on regardless of regulation. So the described situation fits a repressible operon.

Regulation where the default state is transcriptionally on and can be switched off when enough end product is present is repressible control. In repressible operons, the pathway’s end product serves as a corepressor that binds to a repressor protein, enabling it to attach to the operator and block RNA polymerase, turning off transcription. This means the operon remains active while product levels are low, but shuts down once there’s plenty of product to conserve resources. The lac operon, by contrast, is an inducible system that is usually off and turned on by an inducer; constitutive expression would be continuously on regardless of regulation. So the described situation fits a repressible operon.