Discussion
When bacteria are grown in glucose-depleted media containing high concentration of lactose, expression of lac operon genes is activated by
Answer: B
Expression is activated by the dissociation of bound lac repressor from the operator site and binding of cAMP-CAP complex in the CAP site.
Lactose is converted to allolactose, which binds the lac repressor and releases it from the operator, so RNA polymerase is no longer blocked. When glucose is depleted, cAMP levels rise; cAMP binds CAP, and the cAMP-CAP complex binds the CAP site upstream of the promoter, helping RNA polymerase to bind and strongly activating transcription.
Releasing the repressor alone allows only low-level expression. A repressor still bound blocks transcription, and losing cAMP-CAP from its site is what happens when glucose is present.
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