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?

Question

BlackTom AI · Accepted Answer

Low intracellular cAMP in the presence of glucose prevents CAP from activating lac operon transcription.

1) Relevant knowledge points and concepts
- Lac operon regulation involves two key positive/negative controls: LacI repressor (negative control) and the CAP-cAMP complex (positive control, also called catabolite activator protein).
- Lactose (converted to allolactose) inactivates LacI so the operator is de-repressed; however, high-level transcription requires CAP bound to cAMP at the promoter region to recruit RNA polymerase.
- Presence of glucose suppresses adenylate cyclase activity, lowering cAMP (catabolite repression). Also, PTS-dependent inducer exclusion can reduce lactose uptake, but the dominant explanation for low expression with both sugars present is low cAMP/CAP activity.

2) Step-by-step reasoning
- Both glucose and lactose are in the medium, so lactose is available to be converted to allolactose. Allolactose binds LacI, causing it to release the operator; thus repression is relieved.
- However, because glucose is present and being utilized, adenylate cyclase is inhibited; cAMP concentration falls.
- CAP requires cAMP to form CAP-cAMP. Without sufficient cAMP, CAP cannot bind the promoter upstream of the lac operon.
- CAP-cAMP normally facilitates RNA polymerase binding and increases transcription ~50-fold. Without CAP-cAMP, RNA polymerase binds poorly and only basal (very low) transcription occurs.
- Result: lac operon is not highly expressed despite lactose presence.

3) Intermediate “steps” (molecular events)
- Glucose uptake → adenylate cyclase inhibited → ↓ [cAMP].
- ↓ [cAMP] → CAP remains unbound/unactivated → no positive activation at lac promoter.
- LacI inactivated by allolactose → operator free but activation absent → low transcription.

4) Why the correct answer is correct
This explanation directly links glucose-mediated reduction of cAMP to loss of CAP-dependent activation, which is the established mechanism of catabolite repression explaining low lac expression when both sugars are present.

5) Why other explanations are incorrect
- Repressor still bound: unlikely because lactose produces allolactose that inactivates LacI. 
- Promoter mutation or RNAP inability: not supported by the condition “both sugars present”; the canonical regulatory mechanism is catabolite repression.
- Inducer exclusion can contribute but does not explain the key requirement for CAP-cAMP to reach high expression; the provided answer names the central mechanism.

Conclusion: Therefore, the correct answer is: Glucose lowers cAMP levels, preventing CAP from activating transcription of the lac operon. This matches the provided answer.

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?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?

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

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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