The lac operon in E. coli is under dual regulation. Which combination of conditions leads to maximal transcription of the lac operon?

• A Glucose present, lactose absent — repressor inactive, CAP active
• B Glucose absent, lactose present — repressor inactive by allolactose, CAP-cAMP activated at promoter ✓
• C Glucose present, lactose present — both repressor and CAP mechanisms simultaneously active
• D Glucose absent, lactose absent — high cAMP activates CAP at promoter

Explanation

Maximal lac operon transcription requires: (1) absence of glucose → high cAMP → cAMP-CAP complex binds upstream activating sequence, enhancing RNA polymerase binding; AND (2) presence of lactose → allolactose (lac operon inducer) binds and inactivates the lac repressor, freeing the operator. When glucose is present, adenylyl cyclase is inhibited, cAMP falls, CAP is inactive, and transcription is minimal regardless of lactose. Both conditions must be simultaneously met.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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