Q.98 The presence of excess glucose has been known to prevent the induction of
lac operon as well as other operon controlling enzymes involved in carbohydrate
metabolism in E. coli. Which of the following processes define(s) the
phenomenon?
(A) Catabolite repression
(B) Attenuation
(C) Glucose effect
(D) Feedback inhibition
Correct Answer: (A) Catabolite repression
Excess glucose in E. coli prevents induction of the lac operon and other carbohydrate metabolism operons by lowering cAMP levels, which blocks CAP activation of transcription. This ensures glucose is used preferentially over alternatives like lactose.
Option Analysis
Catabolite Repression (A)
High glucose inhibits adenylate cyclase, reducing cAMP and preventing cAMP-CAP complex formation at the lac promoter, thus repressing transcription even with lactose present. This matches the phenomenon described, affecting multiple carbohydrate operons.
Attenuation (B)
Attenuation involves early transcription termination via leader sequence hairpins, mainly in amino acid biosynthetic operons like trp, based on product availability; it does not regulate the lac operon or respond to glucose.
Glucose Effect (C)
“Glucose effect” describes the same diauxic preference for glucose over lactose as catabolite repression, an older term for this cAMP-mediated process in E. coli. While related, standard terminology is catabolite repression.
Feedback Inhibition (D)
This is allosteric enzyme inhibition by end-products (e.g., fructose-1,6-bisphosphate on glycerol kinase), acting post-translationally, not on transcription induction like the query describes.
Excess glucose prevents lac operon induction in E. coli, a key regulatory phenomenon ensuring efficient carbon source use. This process, central to bacterial gene regulation for CSIR NET Life Sciences, prioritizes glucose metabolism over lactose via precise molecular controls.
Lac Operon Basics
The lac operon encodes enzymes (lacZ, lacY, lacA) for lactose breakdown. Normally induced by lactose (via allolactose inactivating LacI repressor), but excess glucose overrides this through global repression of carbohydrate operons.
Catabolite Repression Mechanism
Preferred sugars like glucose lower cAMP by inhibiting adenylate cyclase. Low cAMP prevents CAP binding upstream of the promoter, reducing RNA polymerase recruitment and lac transcription—even with lactose present. This affects multiple operons, explaining the query’s broad scope.
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High glucose → Low cAMP → No CAP activation → Repressed lac operon.
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Glucose depletion → High cAMP → CAP-cAMP binds → Full induction possible.
Key Differences from Other Options
| Process | Mechanism | Applies to Lac Operon? | Glucose Role |
|---|---|---|---|
| Catabolite Repression | cAMP-CAP inhibition of transcription | Yes | Direct trigger |
| Attenuation | Transcription termination in leader | No (trp operon typical) | None |
| Glucose Effect | Synonym for catabolite repression | Yes, but outdated term | Same as (A) |
| Feedback Inhibition | Allosteric enzyme block | No (post-translational) | Indirect |
Relevance for CSIR NET
This question tests operon regulation distinctions. Catabolite repression is the precise answer, as confirmed in exam contexts. Master it alongside lac repressor (negative control) and CAP (positive control) for molecular biology sections.
