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Chapter 15: Problem 9

Explain why catabolite repression is used in regulating the lac operon and describe how it fine-tunes \(\beta\) -galactosidase synthesis.

Short Answer

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Short Answer: Catabolite repression plays a critical role in controlling the lac operon and the synthesis of β-galactosidase in E. coli. It allows the bacterium to prioritize the use of preferred carbon sources, such as glucose, over alternative sources like lactose. This is achieved through the regulation of cAMP and CAP activation, which in turn control the transcription of genes involved in lactose metabolism. When lactose is the sole carbon source, catabolite repression is relieved, and β-galactosidase is synthesized to break down lactose into glucose and galactose. This fine-tuning optimizes energy production and bacterial growth according to the available carbon sources.

Step by step solution

01

Definition of Catabolite Repression and the lac operon

Catabolite repression is a mechanism in which the expression of certain genes, such as those involved in the degradation of carbon sources, is inhibited in the presence of a preferred carbon source (e.g., glucose). In bacteria like E. coli, an important genetic regulation system is the lac operon, which controls the transcription of genes involved in the metabolism of lactose, an alternative carbon source when glucose is absent. The lac operon consists of three structural genes (lacZ, lacY, and lacA) and a regulatory region that includes the promoter and the operator.
02

The Role of cAMP and CAP in Catabolite Repression

In E. coli, catabolite repression is controlled by the intracellular levels of cyclic AMP (cAMP) and the catabolite activator protein (CAP). In the presence of glucose, the levels of cAMP are low, which leads to an inactive CAP. An active CAP is essential for stimulating the transcription of the genes of the lac operon. When glucose levels are low, the levels of cAMP increase, leading to CAP activation and binding to the promoter region of the lac operon, thus initiating the transcription of the genes necessary for lactose metabolism.
03

β-galactosidase and Its Synthesis

The gene lacZ codes for the enzyme β-galactosidase, which is responsible for cleaving lactose into glucose and galactose. The amount of β-galactosidase produced depends on the presence of both lactose and glucose. In the absence of lactose, the lac repressor binds to the operator region, preventing the transcription of lacZ and therefore the production of β-galactosidase.
04

Fine-Tuning β-galactosidase Synthesis

The synthesis of β-galactosidase is fine-tuned by the presence or absence of glucose and lactose in the environment. When glucose is present and lactose is absent, catabolite repression inhibits the production of β-galactosidase. When both glucose and lactose are absent, the lac repressor prevents β-galactosidase synthesis. In the presence of lactose as the sole carbon source, the level of cAMP increases, CAP is activated, and the lac repressor is inactivated, leading to the transcription of lacZ and the production of β-galactosidase. This fine-tuning allows the bacteria to efficiently use available carbon sources, optimizing its energy production and growth.

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Most popular questions from this chapter

Describe which enzymes are required for lactose and tryptophan metabolism in bacteria when lactose and tryptophan, respectively, are (a) present and (b) absent.

In this chapter, we have focused on how prokaryotic and eukaryotic organisms regulate the expression of genetic information. In particular, we discussed both transcriptional and posttranscriptional gene regulation. Based on your knowledge of these topics, answer several fundamental questions: (a) How do we know that bacteria regulate the expression of certain genes in response to the environment? (b) How do we know that bacterial gene clusters are often coordinately regulated by a regulatory region that must be located adjacent to the cluster? (c) What led researchers to conclude that a trans-acting repressor molecule regulates the lac operon? (d) How do we know that promoters and enhancers regulate transcription of eukaryotic genes? (e) How do we know that DNA methylation plays a role in the regulation of eukaryotic gene expression?

Many eukaryotic promoter regions contain CAAT boxes with consensus sequences CAAT or CCAAT approximately 70 to 80 bases upstream from the transcription start site. How might one determine the influence of CAAT boxes on the transcription rate of a given gene?

Compare the control of gene regulation in eukaryotes and prokaryotes at the level of initiation of transcription. How do the regulatory mechanisms work? What are the similarities and dif- ferences in these two types of organisms in terms of the specific components of the regulatory mechanisms? Address how the differences or similarities relate to the biological context of the control of gene expression.

Contrast positive versus negative regulation of gene expression. Describe the role of the repressor in an inducible system and in a repressible system.
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