The lac operon is most strongly transcribed when lactose is present and glucose is absent.
Why does glucose presence reduce lac operon expression even when lactose is abundant?
- A
Glucose inhibits the spliceosome at the lac locus.
- B
Glucose methylates the operator.
- Ccheck_circle
High glucose lowers cAMP levels, so CAP-cAMP cannot bind upstream of the promoter to recruit RNA polymerase efficiently.
- D
Glucose directly binds the lac repressor and activates it.
Explanation
Glucose lowers intracellular cAMP. Without cAMP-bound CAP at the CAP site, RNA polymerase binds the lac promoter weakly (catabolite repression). The cell preferentially uses glucose before switching to lactose.