The presence of excess glucose has been known to prevent the induction of lac operon as well as other operons controlling enzymes involved in carbohydrate metabolism in E. coli. Which of the following processes define(s) the phenomenon?
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Glucose prevents {lac} operon induction by lowering cAMP levels (catabolite repression), which is also called the glucose effect. This ensures glucose is used first before lactose or other sugars.
Step 1: What happens in presence of glucose?
In E. coli, when glucose is abundant, cells preferentially utilize glucose first, repressing the expression of genes required for the metabolism of alternative carbon sources (like lactose, arabinose, maltose). This is an example of diauxic growth.
Step 2: Catabolite repression mechanism
- High glucose $⇒$ low adenylate cyclase activity $⇒$ reduced cAMP levels.
- Low cAMP $⇒$ less cAMP-CAP complex formation.
- Without cAMP-CAP, the lac promoter is poorly activated even if lactose is present.
This regulatory phenomenon is called catabolite repression.
Step 3: Glucose effect
The same process is also historically termed the glucose effect, describing how glucose suppresses the use of other sugars.
Step 4: Eliminate incorrect options
(B) Attenuation: Refers to regulation of transcription via formation of hairpin loops in mRNA (e.g., trp operon), not the case here.
(D) Feedback inhibition: Refers to direct enzyme activity regulation by product inhibition, not operon-level gene repression.
Hence, the correct processes are (A) Catabolite repression and (C) Glucose effect.
