A mutation prevents the lac repressor from recognizing allolactose. In the absence of allolactose, the mutant repressor has the same affinity for the lac operator sites as a normal repressor. How would you expect this mutant repressor to affect the regulation of the lac operon? Assume that no glucose is present.

Question

BlackTom AI · Accepted Answer

Allolactose is the physiological inducer that binds the lac repressor and causes it to release the operator, permitting transcription of the lac operon.

1. Relevant knowledge points and concepts
- lacI encodes the lac repressor which binds the operator and blocks RNA polymerase.
- Allolactose (formed from lactose) binds the repressor, changing its conformation so it detaches from the operator (induction).
- CAP–cAMP activates transcription when glucose is absent, but activation cannot overcome a repressor physically bound to the operator.
- A mutation that prevents the repressor from recognizing allolactose is functionally a “superrepressor” (I^S) that cannot be induced.

2. Step-by-step reasoning
- In wild-type cells with lactose present and no glucose, lactose → allolactose binds repressor → repressor releases operator → CAP–cAMP enhances RNA polymerase binding → operon ON.
- In this mutant, the repressor cannot bind allolactose; therefore even when lactose is present, no inducer binding occurs.
- Because the mutant repressor retains normal affinity for the operator in the absence of allolactose, it remains bound to the operator regardless of external lactose.
- With the repressor bound, RNA polymerase cannot transcribe the operon even though CAP–cAMP would be active in the absence of glucose.

3. Calculations/intermediate steps
- No numerical calculations are required; the argument is mechanistic.

4. Why the correct answer is correct
- The mutant repressor cannot be inactivated by allolactose, so it constitutively represses the operon. Presence or absence of lactose makes no difference; transcription is blocked.

5. Why the other options are incorrect
- “Operon on in presence or absence of lactose” contradicts the repressor’s continued operator binding.
- “Operon off in presence of lactose but on in its absence” is inverted and contradicts the mechanism (inducer presence normally relieves repression).
- “Operon on with lactose but off without” describes wild-type inducible behavior, not the noninducible mutant.

6. Conclusion
Therefore, the operon would be turned off in the presence or absence of lactose in the environment. This matches the provided correct answer.

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