A mutation occurs in the quorum sensing regulation pathway of Vibrio fischeri in which the activated LuxR protein cannot bind to the lux operon. This mutation would cause the lux operon to:

Question

BlackTom AI · Accepted Answer

Quorum sensing in Vibrio fischeri depends on the LuxI/LuxR regulatory circuit: LuxI synthesizes an acyl-homoserine lactone (AHL) autoinducer that accumulates with cell density; AHL binds LuxR, and the AHL–LuxR complex binds the lux operon promoter (the lux box) to activate transcription of luxI and the luciferase genes (luxAB).

1) Relevant knowledge points and concepts
- LuxI produces AHL (autoinducer); AHL diffuses and accumulates.
- AHL binding converts LuxR into its active DNA-binding form.
- Activated LuxR binds the lux operon promoter to recruit RNA polymerase and strongly induce transcription of luxAB (luciferase) and luxI (positive feedback).
- Without LuxR binding, transcriptional activation of the lux operon does not occur.

2) Step-by-step reasoning
- Step 1: In wild-type cells, increasing AHL binds LuxR, forming the active AHL–LuxR complex.
- Step 2: The active complex binds the lux box upstream of the lux operon and activates transcription of luciferase genes (luxAB).
- Step 3: A mutation that renders activated LuxR unable to bind the lux operon breaks Step 2: even though AHL may bind LuxR, the complex cannot attach to DNA.
- Step 4: Without promoter binding, RNA polymerase is not recruited for inducible transcription; the positive-feedback loop (luxI induction) is disrupted, so AHL production falls and induction cannot occur.

3) Calculations
- Not applicable (no numeric computation required).

4) Why the correct answer is correct
- Because DNA binding by activated LuxR is essential for transcriptional activation, loss of binding prevents induction of luxAB. The operon’s strong expression depends on LuxR-mediated activation; without it, the genes encoding luciferase are not transcribed, so luciferase production stops.

5) Why other plausible choices would be incorrect
- "Be constitutively expressed": wrong because constitutive expression would require promoter activity independent of LuxR; the system relies on LuxR activation.
- "Be expressed at low/basal levels": while minimal basal transcription could exist, the physiological luciferase output requires LuxR activation; the key consequence described is loss of luminescence.
- "Be unaffected": incorrect because LuxR binding is required for induction.

6) Conclusion
Because activated LuxR must bind the lux operon to turn on luciferase gene transcription, a mutation preventing LuxR binding will stop luciferase production, confirming the provided answer: Stop production of luciferase completely.

A mutation occurs in the quorum sensing regulation pathway of Vibrio fischeri in which the activated LuxR protein cannot bind to the lux operon. This mutation would cause the lux operon to:单项选择题

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