Which of the following best describes the role of the repressor in the lac operon?

• A. To enhance gene expression
• B. To inhibit transcription of the operon
• C. To facilitate lactose absorption
• D. To promote glucose metabolism

Answer: (B)

The role of the repressor in the lac operon is to inhibit transcription of the operon. In the presence of lactose, the repressor protein, which normally binds to the operator region of the operon and blocks the transcription of downstream genes, is inactivated. This inactivation occurs when lactose is present and converted to allolactose, which binds to the repressor. When allolactose binds, the repressor undergoes a conformational change, leading to its release from the operator. As a result, RNA polymerase can access the promoter region and initiate transcription of genes involved in lactose metabolism, such as lacZ, lacY, and lacA.

This regulatory mechanism allows for efficient use of resources in bacteria. When glucose is scarce and lactose is available, the lac operon is activated to enable the metabolism of lactose. Conversely, when glucose is present, the lac operon is generally off due to the activation of the catabolite repression system and the repressor's binding. Thus, the repressor's primary role is to act as a gatekeeper that prevents unnecessary transcription when lactose is not available.