Question 28:
A researcher engineers a lac operon on a plasmid inactivating all parts of the lac operator and the lac promoter, replacing them with the binding site for the LexA repressor and promoter regulated by LexA. The plasmid is introduced into E.coli cells that have a lac operon with an inactive lacZ gene. Under what conditions will these transformed cells produce β-galactosidase?
Lac operon engineered with LexA promoter produces β-galactosidase during DNA damage. The correct answer for Question 28 is (A).
Correct Answer
Option (A) is correct. The engineered plasmid replaces lac operator/promoter with LexA binding site and LexA-regulated promoter. LexA represses under normal conditions but derepresses upon DNA damage (e.g., UV light), activating RecA* which cleaves LexA, allowing transcription of lacZ for β-galactosidase. Native lacZ is inactive, so only plasmid lacZ expresses.
Option Explanations
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(A) Correct: High DNA-damaging agents like UV generate ssDNA, activating RecA to filament form (RecA*), promoting LexA autocleavage. This inactivates LexA repressor, derepressing the LexA-controlled promoter driving lacZ expression.
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(B) Incorrect: Starvation triggers ppGpp alarmone for stringent response (affecting rRNA), not LexA/SOS pathway. LexA remains bound, blocking transcription.
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(C) Incorrect: Lactose induces native lac operon via allolactose-LacI relief, but plasmid lacks lac operator/promoter. LexA regulation is DNA damage-specific, not lactose-responsive.
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(D) Incorrect: Co-culturing with B. subtilis may trigger competence/quorum sensing, but doesn’t activate E. coli SOS/LexA. No DNA damage signal for RecA*-LexA cleavage.
Conditions Summary
| Condition | Activates LexA Promoter? | β-Galactosidase Produced? | Reason |
|---|---|---|---|
| (A) UV light | Yes | Yes | RecA*-LexA cleavage |
| (B) Starvation | No | No | Stringent response |
| (C) Lactose medium | No | No | LacI irrelevant |
| (D) B. subtilis co-culture | No | No | No SOS trigger |
