An inducible operon, such as the lac operon, is characterized by what regulatory behavior?

• A. It requires an inducer to be active (correct)
• B. It is always on
• C. It is silenced by a constitutive repressor
• D. It has a promoter that is inactive under all conditions

Answer: (A)

Inducible regulation means the operon sits off until an inducer relieves repression. In the lac operon, a repressor protein binds to the operator and blocks transcription. When lactose is present, it is converted to allolactose, which binds the repressor and prevents it from attaching to the operator. With the repressor inactivated, RNA polymerase can initiate transcription of the lac genes, so the operon is expressed only in the presence of the inducer. This is why an inducer is required to activate it. It isn’t always on, and it isn’t silenced by a constitutive repressor, because the repressor’s ability to block transcription is removable by the inducer. The promoter isn’t permanently inactive under all conditions; transcription can occur once repression is lifted.