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Chapter 15: Problem 10

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

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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

01

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.
02

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.
03

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.
04

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.

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Most popular questions from this chapter

What properties demonstrate that the lac repressor is a protein? Describe the evidence that it indeed serves as a repressor within the operon system.

Review the Chapter Concepts list on p. \(285 .\) These all relate to the regulation of gene expression in bacteria. Write a brief essay that discusses why you think regulatory systems evolved in bacteria (i.e., what advantages do regulatory systems provide to these organisms?), and, in the context of regulation, discuss why genes related to common functions are found together in operons.

Contrast the role of the repressor in an inducible system and in a repressible system.

Erythritol is a natural sugar abundant in fruits and fermenting foods. Pathogenic bacterial strains that catabolize erythritol contain four closely spaced genes, all involved in erythritol metabolism. One of the four genes (eryD) encodes a product that represses the expression of the other three genes. Erythritol catabolism is stimulated by erythritol. Present a regulatory model to account for the regulation of erythritol catabolism in such bacterial strains. Does this system appear to be under inducible or repressible control?

In this chapter, we focused on the regulation of gene expression in bacteria. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions? (a) How do we know that bacteria regulate the expression of certain genes in response to the environment? (b) What evidence established that lactose serves as the inducer of a gene whose product is related to lactose metabolism? (c) What led researchers to conclude that a repressor molecule regulates the lac operon? (d) How do we know that the lac repressor is a protein? (e) How do we know that the trp operon is a repressible control system, in contrast to the lac operon, which is an inducible control system?
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