How does the catabolite repression mechanism, involving the cAMP-CAP complex, modulate the expression of the lac operon when both glucose and lactose are present?

Question

BlackTom AI · Accepted Answer

cAMP binds to CAP (also called CRP) to form a complex that enhances RNA polymerase binding at the lac promoter; high glucose lowers adenylate cyclase activity and thus cAMP levels, preventing CAP from binding and reducing lac transcription.

1) Relevant knowledge points and concepts
- Lac operon regulation has two key inputs: the LacI repressor (responds to lactose/allolactose) and catabolite repression via cAMP-CAP (responds to glucose).
- Adenylate cyclase synthesizes cAMP; its activity is inhibited when glucose is abundant.
- The cAMP-CAP complex binds an upstream site and stabilizes RNA polymerase binding, increasing transcription initiation.
- Allolactose (a lactose metabolite) binds LacI and relieves repression, permitting transcription if RNA polymerase can access the promoter.

2) Step-by-step reasoning
- If lactose is present, allolactose will bind LacI → repressor releases the operator → the promoter is not repressed.
- If glucose is also present at high levels, adenylate cyclase activity is low → intracellular cAMP concentration falls.
- Low cAMP means CAP cannot form the cAMP-CAP complex → CAP does not bind the promoter site.
- Without CAP bound, RNA polymerase affinity for the lac promoter is much lower → transcription initiation rate is significantly reduced.
- Net effect: despite operator derepression by lactose, transcription remains low due to lack of positive activation by cAMP-CAP (though a low basal “leaky” transcription may occur).

3) Intermediate steps/calculations
- No numerical calculations are required; the causal chain is: high glucose → low adenylate cyclase activity → low cAMP → no CAP binding → weak RNAP binding → low lac transcription.

4) Why the correct answer is correct
- The chosen option precisely describes the catabolite repression mechanism: high glucose reduces cAMP production, decreasing CAP affinity for the promoter and lowering lac operon transcription even in the presence of lactose.

5) Why other options are incorrect
- Option A: cAMP-CAP does not enhance transcription of LacI; it enhances transcription of the lac operon by helping RNA polymerase bind.
- Option C: cAMP-CAP does not enhance transcription in the presence of glucose; glucose lowers cAMP so the complex is absent.
- Option D: Lac operon transcription is not entirely independent of glucose; catabolite repression links glucose levels to lac expression.

6) Conclusion
The correct answer is: "High glucose levels reduce cAMP production, which decreases the affinity of CAP for the promoter, lowering transcription of the lac operon," which matches the provided answer.

How does the catabolite repression mechanism, involving the cAMP-CAP complex, modulate the expression of the lac operon when both glucose and lactose are present?单项选择题

类似问题

In the presence of lactose but absence of glucose, what happens to cAMP levels and the lac operon?

A researcher grows E. coli in minimal medium containing both glucose and lactose and observes that the lac operon is not highly expressed. Which explanation best accounts for this observation?

A mutation in the lacZ gene alters β-galactosidase so it makes more allolactose compared with a normal β-galactosidase enzyme. How would this mutation affect the expression of the lac operon?

The binding of ___________ to the lac repressor disables the lac repressor to bind to the site, thereby transcription.

On the chromosome, CAP is mutated such that it no longer produces a functional CAP protein, and the operator is mutated such that it no longer binds to the repressor. On the plasmid, lacI is mutated so that it no longer produces a functional repressor protein. Lactose and glucose are both absent.

Match the following states of nutrient availability with the corresponding state of the lac operon during gene expression regulation: 1: High glucose present, no lactose present 2: High glucose present, lactose also present 3: Low glucose present, high lactose present

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