How would a cap⁻ mutation that produces an inactive CAP protein affect transcriptional control of the lac operon?

Verified step by step guidance

1. Understand the role of CAP (Catabolite Activator Protein) in the lac operon: CAP, when bound to cAMP, enhances the transcription of the lac operon by facilitating the binding of RNA polymerase to the promoter.
2. Recognize that a cap⁻ mutation results in an inactive CAP protein, which cannot bind to cAMP and therefore cannot assist in the transcriptional activation of the lac operon.
3. Consider the conditions under which the lac operon is normally activated: In the presence of lactose and absence of glucose, cAMP levels are high, allowing CAP to bind to the promoter and enhance transcription.
4. Analyze the impact of the cap⁻ mutation: Without active CAP, even in the presence of lactose and absence of glucose, the lac operon will have reduced transcription because RNA polymerase binding is less efficient.
5. Conclude that the cap⁻ mutation leads to decreased expression of the lac operon genes, as the positive regulation by CAP is impaired, affecting the cell's ability to metabolize lactose efficiently.

Key Concepts

CAP Protein Function

The CAP (catabolite activator protein) is a transcription factor that enhances the expression of certain genes, including the lac operon, in response to low glucose levels. When glucose is scarce, CAP binds to cyclic AMP (cAMP), forming a complex that promotes RNA polymerase binding to the promoter of the lac operon, facilitating transcription.

Lac Operon Regulation

The lac operon is a set of genes in E. coli that are involved in the metabolism of lactose. Its expression is regulated by the presence of lactose and glucose. In the absence of glucose, the CAP-cAMP complex binds to the lac promoter, enhancing transcription, while the presence of glucose inhibits this process, demonstrating a classic example of catabolite repression.

Mutation Effects on Gene Expression

A mutation that leads to an inactive CAP protein would disrupt the normal regulatory mechanism of the lac operon. Without functional CAP, the operon would be less efficiently activated in low glucose conditions, resulting in reduced transcription of the genes necessary for lactose metabolism, even when lactose is present.