Question

Predict the effect on the inducibility of the lac operon of a mutation that disrupts the function of (a) the crp gene, which encodes the CAP protein, and (b) the CAP-binding site within the promoter.

Short answer

Answer: Mutations in the crp gene and the CAP-binding site are expected to negatively impact the inducibility of the lac operon. A mutation in the crp gene would result in impaired production of functional CAP protein, leading to reduced activation of the lac operon. A mutation in the CAP-binding site would affect the binding of the CAP-cAMP complex and hinder the recruitment of RNA polymerase, also leading to decreased transcription of the lac operon. Both mutations would result in decreased lactose metabolism.

Step-by-step solution

Understanding the lac operon

The lac operon is a group of genes involved in lactose metabolism in bacteria. It includes three structural genes (lacZ, lacY and lacA), a promoter (P_lac) and an operator (O_lac). The regulation of the lac operon involves several components such as the lac repressor protein, the CAP protein, and the CAP-binding site.

Function of the CAP protein and CAP-binding site

Catabolite activator protein (CAP) is a regulatory protein essential for the activation of the lac operon. When the glucose levels are low, cAMP level increases, and cAMP binds to CAP, forming a CAP-cAMP complex. This complex binds to the CAP-binding site situated within the promoter. Binding of the CAP-cAMP complex enhances RNA polymerase binding and increases transcription of the lac operon.

Predict the effect of a mutation in the crp gene

A mutation in the crp gene, which encodes the CAP protein, would disrupt the production of functional CAP protein. When functional CAP protein is absent, the CAP-cAMP complex cannot be formed, and it can't bind to the CAP-binding site. As a consequence, the activation of the lac operon under low glucose condition will be impaired. Thus, the inducibility of the lac operon would be reduced or lost, resulting in decreased lactose metabolism.

Predict the effect of a mutation in the CAP-binding site

A mutation in the CAP-binding site within the promoter would affect the binding of the CAP-cAMP complex to the promoter. In this case, even if the CAP protein and cAMP levels are normal, the complex cannot bind effectively to the mutated CAP-binding site. This would result in weakened or abolished recruitment of RNA polymerase, leading to decreased transcription of the lac operon. Consequently, the inducibility of the lac operon would be reduced or lost. In conclusion, mutations in both (a) the crp gene and (b) the CAP-binding site would negatively impact the inducibility of the lac operon, resulting in decreased lactose metabolism.