How does the lac operon regulate gene expression in response to lactose?

• A. Allolactose activates LacI repressor, repressing transcription.
• B. Allolactose inactivates the LacI repressor, permitting transcription; in low glucose, the CAP-cAMP complex activates transcription, leading to lacZYA expression.
• C. Lactose inhibits RNA polymerase directly; transcription stops.
• D. Lactose is the primary activator of the LacI repressor.

Answer: (B)

The main idea tested is how the lac operon turns on in response to lactose through removal of repression and, when glucose is scarce, enhancement by an activator. When lactose is present, it is converted to allolactose, which binds to the LacI repressor and prevents LacI from blocking the operator. This derepression allows RNA polymerase to transcribe the lacZYA genes. If glucose is also low, cyclic AMP levels rise and CAP binds cAMP; the CAP-cAMP complex then helps RNA polymerase bind to the promoter, further boosting transcription. Together, lactose presence and low glucose maximize lac operon expression. The other statements misrepresent the roles: allolactose does not activate the repressor, lactose does not directly inhibit RNA polymerase, and lactose is not the activator of LacI repressor.