Question

What is the benefit of the repressor gene being constitutively expressed?

• A. The repressor protein provides a feedback loop preventing the lac operon from being transcribed in the absence of glucose.
• B. The bacteria does not want to expend energy creating lactose metabolizing enzymes if lactose is not available.
• C. The repressor protein acts as a lactose sensor, and recruits RNA polymerase only in the presence of lactose.
• D. The operator needs either a constitutively expressed activator or repressor protein binding to it to regulate transcription.

Answer 1

The correct option is B The bacteria does not want to expend energy creating lactose metabolizing enzymes if lactose is not available.

The main regulatory gene that acts on the lac operon is a repressor, which is constitutively expressed. In the absence of lactose, the repressor protein binds to the lac operon operator, where it blocks RNA polymerase from transcribing the lac operon.
The bacteria does not want to expend energy creating lactose metabolizing enzymes if lactose is not available.