Question

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

Short answer

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

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.

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.

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.

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.