Q.73. Which one of the following conditions favors maximum expression of lac operon genes in E. coli?
(A) Glucose-low, lactose-low, cAMP-high
(B) Glucose-high, lactose-low, cAMP-high
(C) Glucose-low, lactose-high, cAMP-high
(D) Glucose-high, lactose-high, cAMP-low
The lac operon in E. coli controls lactose metabolism genes through negative control by the lac repressor and positive control by the CAP-cAMP complex. Maximum expression occurs when lactose is present to inactivate the repressor and glucose is absent to elevate cAMP for CAP activation.
Correct Answer
Option (C) Glucose-low, lactose-high, cAMP-high favors maximum expression of lac operon genes. Low glucose raises intracellular cAMP levels, enabling the CAP-cAMP complex to bind the promoter and boost RNA polymerase recruitment. High lactose converts to allolactose, which binds the repressor, releasing it from the operator to allow full transcription.
Option Analysis
(A) Glucose-low, lactose-low, cAMP-high
Low glucose activates CAP-cAMP for positive regulation, but low lactose keeps the repressor bound to the operator, blocking transcription. Expression remains minimal despite CAP activation.
(B) Glucose-high, lactose-low, cAMP-high
High glucose typically lowers cAMP, contradicting the “cAMP-high” label, but even if cAMP were high, low lactose maintains repressor binding. No significant expression occurs due to operator blockage.
(C) Glucose-low, lactose-high, cAMP-high
This optimal state combines CAP-cAMP enhancement from low glucose with repressor inactivation from high lactose. It results in the highest transcription rate of lacZ, lacY, and lacA genes.
(D) Glucose-high, lactose-high, cAMP-low
High lactose removes the repressor, allowing basal transcription, but high glucose keeps cAMP low, preventing CAP activation. Expression is leaky and low, not maximal.
Regulation Mechanism
The lac operon exemplifies inducible and catabolite-repressed systems. In glucose absence, adenylate cyclase produces cAMP; CAP-cAMP binds upstream of the promoter. Allolactose from lactose inactivates the repressor, synergizing for 1000-fold induction over repressed levels.
This setup ensures E. coli prioritizes glucose, activating lactose genes only when needed, optimizing energy use in varying environments.
