Q.15 When bacteria are grown in glucose-depleted media containing high concentration of lactose, expression of lac operon genes is activated by

In glucose-depleted media with high lactose concentration, lac operon genes achieve high-level expression through two key events: the lac repressor dissociates from the operator due to allolactose (a lactose derivative), and the cAMP-CAP complex binds to the CAP site because low glucose elevates cAMP levels.​

Option Analysis

(A) Incorrect. Lac repressor binding to the operator blocks transcription, so it cannot activate expression; cAMP-CAP binding alone is insufficient without repressor release.​

(B) Correct. High lactose inactivates the repressor (via allolactose binding, causing dissociation from operator), while glucose depletion raises cAMP, enabling CAP-cAMP to bind the CAP site and boost RNA polymerase recruitment for full activation.​

(C) Incorrect. Repressor dissociation allows basal transcription, but without cAMP-CAP binding (which requires glucose depletion), expression remains low rather than activated.​

(D) Incorrect. cAMP-CAP dissociation occurs in high-glucose conditions and represses expression; activation requires its binding, not dissociation.​

The lac operon activation in glucose-depleted media containing high lactose represents a classic example of dual regulation in E. coli, ensuring efficient lactose metabolism only under optimal conditions. This mechanism, pivotal for CSIR NET Life Sciences, combines negative control (repressor) and positive control (cAMP-CAP).​

Core Regulation Mechanism

Low glucose triggers adenylate cyclase to produce high cAMP, which binds CAP to form the cAMP-CAP complex; this binds the CAP site upstream of the promoter, enhancing RNA polymerase affinity. Simultaneously, high lactose converts to allolactose, which binds the lac repressor (LacI), inducing a conformational change that dissociates it from the operator, freeing the promoter for transcription. Result: maximal lacZ, lacY, lacA expression for β-galactosidase, permease, and transacetylase.​

Conditions for Full Activation

• Glucose-depleted, high lactose: High expression (repressor off, CAP on).​

• High glucose, high lactose: Low expression (repressor off, but CAP off).​

• Glucose-depleted, no lactose: No expression (CAP on, but repressor bound).​

• High glucose, no lactose: Repressed (both repressor bound, CAP off).​