7. E. coli is being grown in a medium containing both glucose and galactose. On depletion of glucose, expression of β-galactosidase will
(1) remain unchanged
(2) increase
(3) decrease
(4) initially decrease and then increase

Efficient sugar metabolism is vital for the survival and growth of bacteria like Escherichia coli (E. coli). The ability of E. coli to adapt gene expression in response to available nutrients is a classic example of molecular regulation. One of the most studied enzymes in this context is β-galactosidase, which plays a critical role in lactose metabolism. But how does the expression of β-galactosidase change when E. coli is grown in a medium containing both glucose and galactose and then glucose becomes depleted? This article explores the molecular mechanisms behind this regulation and provides the correct answer to this important question in microbial physiology.

Understanding β-Galactosidase and the Lac Operon

β-Galactosidase is encoded by the lacZ gene, part of the lac operon in E. coli. Its main function is to break down lactose into glucose and galactose, allowing the bacterium to utilize lactose as a source of energy. The lac operon is a group of genes that are transcribed together and are tightly regulated to ensure energy efficiency.

Key Regulatory Elements

• Lac Repressor: Binds to the operator region, blocking transcription of the lac operon when lactose is absent.

• CAP (Catabolite Activator Protein): Enhances transcription of the lac operon when glucose is absent and cAMP levels are high.

• Catabolite Repression: A regulatory mechanism that suppresses the lac operon when glucose is available, even if other sugars are present.

E. coli Sugar Preference: Glucose vs. Galactose

E. coli prefers glucose over other sugars because it is easier to metabolize. When both glucose and galactose are present, the bacterium will use glucose first. This preference is enforced by catabolite repression, which keeps the lac operon—and thus β-galactosidase expression—at low levels as long as glucose is available.

What Happens When Glucose Is Depleted?

When glucose is depleted in a medium that still contains galactose, several regulatory changes occur:

• cAMP Levels Rise: The absence of glucose increases the concentration of cyclic AMP (cAMP) in the cell.

• CAP Activation: cAMP binds to CAP, activating it. The CAP-cAMP complex binds near the lac promoter, making it easier for RNA polymerase to initiate transcription.

• Lac Operon Activation: If an inducer (such as lactose or a lactose analog) is present, the lac repressor is inactivated, and the lac operon is transcribed at a higher rate.

• β-Galactosidase Expression Increases: As a result, the production of β-galactosidase increases, enabling E. coli to metabolize alternative sugars.

Correct Answer

When E. coli is grown in a medium containing both glucose and galactose, and glucose becomes depleted, the expression of β-galactosidase will:

(2) increase

This increase is due to the lifting of catabolite repression, allowing the lac operon to be fully expressed and β-galactosidase to be produced in greater amounts.

Molecular Mechanism: Step-by-Step

1. Glucose Present:

   • Low cAMP, inactive CAP.

   • Lac operon repressed, low β-galactosidase expression.

2. Glucose Depleted:

   • High cAMP, active CAP.

   • CAP-cAMP complex binds to the promoter region.

   • If an inducer is present, the lac repressor is inactivated.

   • RNA polymerase efficiently transcribes the lac operon.

   • β-Galactosidase expression increases.

Biological Significance

• Energy Efficiency: E. coli conserves resources by producing β-galactosidase only when necessary.

• Metabolic Flexibility: The bacterium can quickly switch from glucose to other sugars, ensuring survival in diverse environments.

• Classic Example of Gene Regulation: The lac operon is a model system for understanding how cells regulate gene expression in response to environmental cues.

Summary Table: β-Galactosidase Expression in Different Sugar Conditions

Frequently Asked Questions

Q: Does galactose directly induce the lac operon?
A: No, the lac operon is primarily induced by lactose or its analogs. However, the regulatory logic is similar for other sugars when glucose is absent.

Q: Can β-galactosidase be expressed when glucose is present?
A: Only at very low, basal levels due to catabolite repression.

Q: Why does E. coli repress the lac operon in the presence of glucose?
A: To prioritize the most energy-efficient sugar and avoid unnecessary enzyme production.

Conclusion

When E. coli is grown in a medium containing both glucose and galactose, the expression of β-galactosidase remains low while glucose is present due to catabolite repression. Upon glucose depletion, this repression is lifted, and β-galactosidase expression increases, allowing the bacterium to adapt and utilize available galactose or other sugars. This elegant regulatory mechanism ensures optimal energy usage and highlights the sophistication of bacterial gene regulation.

Keywords: β-galactosidase expression, E. coli, glucose depletion, galactose, lac operon, catabolite repression, cAMP, CAP, gene regulation, lactose metabolism, bacterial adaptation, RNA polymerase, lac repressor, metabolic flexibility, inducible operon, sugar metabolism, molecular biology, operon systems