Which of the following is a characteristic of the lac repressor protein in the lac operon?

• A. It binds to the operator region to inhibit transcription.
• B. It enhances the binding of RNA polymerase to the promoter.
• C. It directly interacts with the CAP protein to regulate gene expression.
• D. It is activated by the presence of lactose.

Answer: (A)

Verified step by step guidance

1. Understand the role of the lac repressor protein in the lac operon system. The lac operon is a set of genes involved in lactose metabolism in bacteria, and the lac repressor is a protein that regulates these genes.
2. Identify the function of the lac repressor. It binds to the operator region of the lac operon, which is a segment of DNA located near the promoter. This binding prevents RNA polymerase from transcribing the downstream genes necessary for lactose metabolism.
3. Consider the conditions under which the lac repressor is active. In the absence of lactose, the repressor binds to the operator, inhibiting transcription. When lactose is present, it binds to the repressor, causing a conformational change that reduces the repressor's affinity for the operator, allowing transcription to proceed.
4. Differentiate between the roles of the lac repressor and other proteins involved in the lac operon, such as the CAP protein. The CAP protein enhances the binding of RNA polymerase to the promoter in the presence of cAMP, which is a separate regulatory mechanism from the repressor's function.
5. Conclude that the primary characteristic of the lac repressor protein is its ability to bind to the operator region to inhibit transcription, which is the correct answer to the problem.