A deletion of the lacP (promoter) region in the lac operon would result in which of the following?

Question

BlackTom AI · Accepted Answer

Promoters are DNA sequences required for RNA polymerase binding and transcription initiation; removing the lacP promoter therefore prevents RNA polymerase from recognizing and initiating transcription of the lac operon.

1. Relevant knowledge points and concepts
- The lac operon requires a promoter (lacP) to recruit RNA polymerase (with sigma factor) to the -35 and -10 consensus sequences; initiation of mRNA synthesis depends on this interaction.
- The operator (lacO) is the DNA binding site for the lac repressor (LacI) and is functionally distinct from the promoter.
- Positive regulation via CAP/cAMP enhances promoter activity but still requires a functional promoter sequence.
- “Leaky” or basal transcription requires at least part of a promoter; deletion of the promoter removes that core requirement.

2. Step-by-step reasoning
- Step 1: RNA polymerase locates promoters via specific sequences (−35/−10). lacP contains these sequences for the lac operon.
- Step 2: Deletion of lacP removes the binding site motifs needed for RNA polymerase recognition.
- Step 3: Without binding, RNA polymerase cannot form an open complex or begin transcription at the lac operon start site.
- Step 4: Therefore, transcription of lacZ, lacY, and lacA will be effectively abolished regardless of lactose or repressor state.

3. Intermediate logical steps
- Even if the repressor were absent (de-repressed) or CAP/cAMP were present (activated), these factors cannot recruit RNA polymerase if its binding site is gone.
- While transcription read-through from an upstream promoter is theoretically possible, the standard interpretation of promoter deletion in this question assumes loss of native initiation and thus loss of operon transcription.

4. Why the correct answer is correct
- The statement “RNA polymerase would be unable to initiate transcription of the lac operon” directly follows from the loss of the promoter sequences required for polymerase binding and initiation.

5. Why other options are incorrect
- “The lac operon would be constitutively expressed” is false because constitutive expression requires an intact promoter to drive continuous transcription.
- “The operon would still be expressed, but at very low levels” is incorrect because basal expression depends on promoter elements; deletion removes them and prevents initiation (unless unlikely read-through, not assumed).
- “The lac repressor would no longer bind to the operator region” is wrong because repressor binding depends on the operator sequence, not the promoter; deleting lacP does not necessarily remove lacO.

6. Conclusion
The correct result of deleting lacP is that RNA polymerase would be unable to initiate transcription of the lac operon, matching the provided answer.

A deletion of the lacP (promoter) region in the lac operon would result in which of the following?Single choice

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