USA Biology Olympiad (USABO) Practice Exam

What happens if the lac repressor of E. coli is mutated to prevent binding to the operator?

1. Constant expression of lac genes

2. Complete inhibition of lac genes

3. The result depends on the concentration of glucose

4. Immediate lac operon activation

The correct answer is: The result depends on the concentration of glucose

If the lac repressor of E. coli is mutated such that it can no longer bind to the operator, the lac operon will not be subject to repression under normal circumstances. This means that the genes responsible for the metabolism of lactose will be expressed continuously, as there is no functional repressor to inhibit their transcription. However, the expression of these genes is also regulated by the presence of glucose within the cell. In the absence of glucose, cAMP levels rise, leading to the activation of catabolite activator protein (CAP), which enhances the transcription of the lac operon. This situation leads to the effective expression of lac genes. Conversely, when glucose is present, the cAMP levels decrease, resulting in diminished activation by CAP and thus reducing the expression of the lac operon despite the repressor's inability to bind. Therefore, while the mutation of the lac repressor results in the constant potential for lac genes to be expressed, the actual expression also heavily depends on the concentration of glucose present in the environment, leading to the conclusion that the outcome hinges on glucose levels.