Question

Catabolite repression in bacteria is regulated by the concentration of

• A. amino acids
• B. glucose
• C. messenger RNA
• D. lactose

Hint:

Glucose presence = low cAMP = no CRP activation = catabolite repression. Glucose absence = high cAMP = CRP–cAMP activates alternative operons.

Answer 1

The Correct Option is B

Step 1: Concept of catabolite repression. In bacteria, especially E. coli, when glucose is available, it is preferentially used as the carbon source. The presence of glucose represses the expression of operons (like the lac operon) that metabolize alternative sugars. This is called catabolite repression. Step 2: Molecular mechanism. Catabolite repression operates through the cAMP–CRP (catabolite activator protein) system. \(\bullet\) When glucose is abundant, adenylate cyclase activity is low \(⇒\) cAMP levels fall \(⇒\) CRP–cAMP complex does not form \(⇒\) transcription of alternative operons is repressed. \(\bullet\) When glucose is scarce, cAMP levels rise, CRP–cAMP complex binds DNA, and transcription of alternative metabolic pathways (e.g. lactose utilization) is activated. Step 3: Evaluating options. (A) Amino acids regulate amino acid biosynthesis operons (e.g. tryptophan operon), not catabolite repression. ❌ (B) Glucose is the key regulator of catabolite repression. ✔️ (C) mRNA is the product of transcription and not the regulator. ❌ (D) Lactose is an inducer of the lac operon, but catabolite repression overrides lactose induction when glucose is present. ❌ Hence, the answer is (B) glucose.