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

Question

BlackTom AI · Accepted Answer

When glucose is absent, adenylate cyclase becomes active and synthesizes cAMP, so cAMP levels rise.

1) Relevant concepts:
- Lac operon regulation involves two signals: the lac repressor (responding to lactose/allolactose) and catabolite activator protein (CAP) responding to cAMP.
- Presence of lactose produces allolactose, which binds the lac repressor and prevents it from binding the operator (de-repression).
- Low glucose leads to high intracellular cAMP; cAMP binds CAP, and the CAP–cAMP complex binds upstream of the lac promoter to enhance RNA polymerase recruitment (catabolite activation).

2) Step-by-step reasoning:
- Condition: lactose present, glucose absent.
- Step A: Lactose is converted to allolactose (the true inducer). Allolactose binds the lac repressor (LacI), causing a conformational change so LacI cannot bind the operator → operator is free.
- Step B: Absence of glucose activates adenylate cyclase → intracellular cAMP concentration increases.
- Step C: cAMP binds CAP (CRP), forming the CAP–cAMP complex. This complex binds the CAP site near the lac promoter.
- Step D: CAP–cAMP binding facilitates RNA polymerase binding and transcription initiation, producing high levels of lac operon mRNA.

3) Intermediate logical steps shown above (A–D) trace how both de-repression and positive activation are achieved under these conditions.

4) Why the correct answer is correct:
Because lactose inactivates the repressor (permitting transcription) and lack of glucose elevates cAMP which allows CAP to bind and strongly activate transcription, the lac operon is highly expressed with CAP bound to the promoter.

5) Why the other options are incorrect:
- "cAMP low, but lac operon strongly expressed": low cAMP (from glucose presence) prevents CAP activation, so expression is weak even if repressor were absent.
- "cAMP high, but lac operon is repressed by the lac repressor": contradicts lactose presence — lactose (allolactose) inactivates the repressor, so it cannot repress.
- "cAMP low, and the lac operon is repressed": describes high-glucose/low-lactose conditions, not the given scenario.

6) Conclusion:
Therefore, cAMP levels are high, and CAP binds to the promoter to enhance transcription of the lac operon — matching the provided correct answer.

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

类似问题

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?

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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