6. The lac operon in E. coli. is controlled by both the lac repressor and the catabolite activation protein CAP. In an in vivo experiment with lac operon, the following observations are made:
(A) cAMP levels are high
(B) Repressor is bound With allolactose
(C) CAP is interacting With RNA polymerase
Which one of the following conclusions is most appropriate based on the above observations?
(1) Glucose and lactose are present,
(2) Glucose is present and lactose absent
(3) Both are absent
(4) Glucose is absent and lactose is present

The lac operon in Escherichia coli is a textbook example of how bacteria regulate gene expression in response to environmental changes. This operon allows E. coli to metabolize lactose efficiently, but only under specific conditions. The regulation involves both negative and positive controls, integrating signals about glucose and lactose availability. Let’s analyze what it means when you observe high cAMP levels, a repressor bound with allolactose, and CAP interacting with RNA polymerase in vivo.

Key Regulatory Components of the Lac Operon

• Lac Repressor: Binds to the operator to block transcription when lactose is absent.

• Allolactose: An isomer of lactose, acts as the true inducer by binding to the repressor and inactivating it.

• CAP (Catabolite Activator Protein): Binds upstream of the promoter when cAMP is high, enhancing RNA polymerase binding and transcription.

• cAMP (Cyclic AMP): Its concentration is inversely related to glucose levels; high cAMP means glucose is scarce.

Interpreting the Observations

A. High cAMP Levels

• High cAMP signals that glucose is absent or at very low levels.

• CAP binds cAMP, forming a complex that attaches near the lac promoter, boosting transcription.

B. Repressor Bound With Allolactose

• Allolactose is present only when lactose is available.

• When allolactose binds the repressor, the repressor changes shape and releases the operator, allowing transcription to proceed.

C. CAP Interacting With RNA Polymerase

• CAP, when activated by cAMP, binds upstream of the promoter and recruits RNA polymerase.

• This interaction significantly increases the transcription of lac operon genes.

What Do These Observations Mean?

When all three of these conditions are met:

• Glucose is absent (high cAMP, active CAP)

• Lactose is present (allolactose inactivates the repressor)

• The lac operon is fully switched on (CAP helps RNA polymerase bind efficiently)

This is the only scenario where E. coli maximally expresses the lac operon, producing all the enzymes needed for lactose metabolism.

Option Analysis

Let’s match these observations to the provided options:

1. Glucose and lactose are present

   • If glucose were present, cAMP would be low and CAP would not be active.

2. Glucose is present and lactose absent

   • If lactose were absent, the repressor would not be bound to allolactose and would block transcription.

3. Both are absent

   • If both sugars were absent, cAMP would be high but the repressor would not be inactivated (no allolactose), so transcription would be blocked.

4. Glucose is absent and lactose is present

   • High cAMP (glucose absent), allolactose-bound repressor (lactose present), and active CAP-RNA polymerase interaction all occur only in this condition.

The most appropriate conclusion is:

Glucose is absent and lactose is present.

Why Is This Regulation So Efficient?

E. coli prefers glucose because it is easier to metabolize. Only when glucose is depleted and lactose is available does the cell activate the lac operon. This dual regulation prevents wasteful production of lactose-metabolizing enzymes and ensures optimal energy utilization.

Summary Table: Lac Operon Regulatory States

Conclusion

High cAMP levels, a repressor bound with allolactose, and CAP-RNA polymerase interaction together indicate that glucose is absent and lactose is present. This is the precise condition in which the lac operon is fully activated, demonstrating the elegant regulatory logic that allows E. coli to adapt to its nutritional environment.

Keywords: lac operon, E. coli, cAMP, CAP, allolactose, glucose, lactose, gene regulation, RNA polymerase, transcription, operon activation, metabolic regulation, positive regulation, negative regulation, bacterial metabolism, inducible operon.