In E. coli, the lac operon regulates the genes necessary for lactose metabolism. For the lac operon to achieve its highest level of transcription, what environmental conditions must be met?
Show answer and explanation
The answer
Correct answer: B. Lactose must be present to inactivate the repressor, and glucose must be absent to activate CAP.
Tested concept: The lac and trp operons
Explanation
The lac operon is under dual control. For maximal transcription, two conditions must be met: 1) Lactose must be present. Lactose (or its isomer allolactose) acts as an inducer, binding to the lac repressor and preventing it from binding to the operator, thus allowing RNA polymerase to transcribe the genes. 2) Glucose must be absent. When glucose levels are low, cyclic AMP (cAMP) levels are high. cAMP binds to the catabolite activator protein (CAP), forming a cAMP-CAP complex. This complex then binds to the CAP binding site near the promoter, enhancing the binding of RNA polymerase and significantly increasing transcription. Therefore, high lactose and low glucose lead to maximal lac operon expression. Distractor 1 is incorrect because the absence of lactose would keep the repressor bound, preventing transcription, and the presence of glucose would keep CAP inactive, further reducing expression. Distractor 2 is incorrect because the presence of glucose, even with lactose, would lead to catabolite repression, keeping cAMP levels low and CAP inactive, thus preventing maximal transcription. Distractor 3 is incorrect because while lactose presence is necessary for induction, it is not sufficient for maximal transcription; the absence of glucose is also required for CAP activation and full enhancement of transcription.
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