Q.71 Catabolite repression in bacteria is regulated by the concentration of
(A) amino acids
(B) glucose
(C) messenger RNA
(D) lactose
The correct answer is (B) glucose.
Catabolite repression enables bacteria like E. coli to prioritize glucose metabolism over alternative sugars such as lactose. High glucose levels reduce intracellular cAMP, preventing the cAMP-CRP complex from activating transcription of catabolic operons like lac.
Option Analysis
(A) Amino acids
Amino acids regulate nitrogen metabolism pathways like the arginine operon through attenuation or repression, not catabolite repression of carbohydrate utilization.
(B) Glucose
Glucose triggers catabolite repression by inhibiting adenylate cyclase, lowering cAMP levels. This blocks CRP activation needed for operons like lac, ensuring glucose preference.
(C) Messenger RNA
mRNA serves as an intermediary for gene expression but does not regulate catabolite repression; regulation occurs at transcription via cAMP-CRP.
(D) Lactose
Lactose induces the lac operon by binding the repressor, but glucose overrides this through catabolite repression, preventing lac expression.
Introduction
Catabolite repression in bacteria regulated by glucose concentration is a key regulatory mechanism where glucose suppresses genes for alternative carbon sources like lactose. This ensures efficient energy use in environments with mixed sugars, a core concept for CSIR NET Life Sciences aspirants studying bacterial gene regulation.
Mechanism Overview
When glucose is abundant, it lowers cAMP via adenylate cyclase inhibition. The cAMP-CRP complex fails to bind lac operon promoters, repressing β-galactosidase and permease synthesis even with lactose present. Low glucose raises cAMP, enabling CRP activation for full operon expression.
Biological Importance
This diauxic growth pattern—glucose first, then lactose—optimizes bacterial fitness by avoiding wasteful enzyme production. In E. coli, it coordinates PTS transport with global carbon control.
Exam Relevance
CSIR NET questions test this via MCQs on lac operon regulation. Glucose as the repressor distinguishes it from inducer exclusion or Gram-positive CcpA pathways.
