Catabolite repression scenario: In the presence of glucose, which regulatory event occurs at the lac promoter region?
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ABinding of CAP–cAMP complex to the promoter area decreases markedly
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BlacZ expression is strongly increased by glucose alone
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CIntracellular cAMP levels increase when glucose is abundant
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DNone of the above changes occur with glucose
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ELac repressor automatically dissociates from the operator
Answer
Correct Answer: Binding of CAP–cAMP complex to the promoter area decreases markedly
Explanation
Introduction / Context: Glucose availability affects the lac operon by modulating cAMP levels and CAP binding, a process called catabolite repression. This ensures E. coli preferentially uses glucose before lactose.
Given Data / Assumptions:
- High glucose → low adenylate cyclase activity → low cAMP.
- Low cAMP → less CAP–cAMP complex bound at the promoter.
- Lac repressor responds to lactose/allolactose, not directly to glucose.
Concept / Approach: Transcription of lac genes requires both repressor relief (induction by lactose/allolactose) and CAP activation (high cAMP). Glucose specifically depresses cAMP, weakening CAP activation.
Step-by-Step Solution: Relate glucose to cAMP: glucose present → cAMP decreases. Relate cAMP to CAP binding: low cAMP → less CAP–DNA binding. Choose the option describing decreased CAP–cAMP binding.
Verification / Alternative check: Classic diauxic growth curves show lag in lactose utilization until glucose is depleted, reflecting CAP–cAMP control.
Why Other Options Are Wrong: Glucose does not increase cAMP; glucose alone does not upregulate lacZ; LacI does not automatically dissociate due to glucose.
Common Pitfalls: Confusing repressor induction with catabolite repression; assuming glucose directly affects LacI binding.
Final Answer: Binding of CAP–cAMP complex to the promoter area decreases markedly.