A mutation in the lac operator region (O) that prevents the binding of the lac repressor (LacI) would result in which of the following?

Question

BlackTom AI · Accepted Answer

The lac operator (O) is the specific DNA sequence where the LacI repressor binds to block RNA polymerase; if LacI cannot bind, transcription is not blocked and the structural genes are transcribed continuously.

1) Relevant knowledge points and concepts
- The lac operon has three key regulatory components: the promoter/operator region (where RNA polymerase and LacI bind), the LacI repressor protein, and the inducer (allolactose) that inactivates LacI. 
- When LacI binds the operator it prevents RNA polymerase from transcribing lacZYA; when allolactose binds LacI, the repressor releases the operator, allowing transcription (induction). 
- Positive regulation by CAP-cAMP modulates transcription level based on glucose, but operator binding determines whether repression can occur.

2) Step-by-step reasoning
- If a mutation in O prevents LacI binding, the physical block to RNA polymerase is removed permanently. 
- LacI cannot repress transcription regardless of whether allolactose is present. Therefore the operon is produced continuously (constitutively). 
- Presence or absence of lactose no longer controls repression because the repressor cannot access its site. 
- Note: the absolute level of transcription can still be influenced by catabolite repression (glucose/CAP-cAMP), but the operon is no longer switchable by LacI.

3) Calculations
- No numerical calculations are required for this conceptual genetics question.

4) Why the correct answer is correct
- Constitutive expression follows directly from loss of repressor binding: without operator-mediated repression, RNA polymerase can transcribe the lac genes irrespective of inducer presence.

5) Why other options are incorrect
- "Complete shutdown even in absence of lactose": opposite of expected; preventing repressor binding cannot cause shutdown. 
- "Repressed even in presence of lactose": if LacI cannot bind the operator, it cannot repress, so repression cannot persist. 
- "Induction only in presence of lactose": induction normally requires removal of repressor; here repression is permanently absent, so induction is not required and transcription is not restricted to lactose presence.

6) Conclusion
This reasoning confirms the correct answer: Constitutive expression of the lac operon, regardless of the presence of lactose.

A mutation in the lac operator region (O) that prevents the binding of the lac repressor (LacI) would result in which of the following?单项选择题

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