Which of the following accurately describes the binding of lac repressor to the lac operon?

Question

BlackTom AI · Accepted Answer

The lac repressor is a sequence-specific DNA-binding protein that recognizes and binds the lac operator sequence via a helix–turn–helix motif, preventing RNA polymerase from transcribing the lacZYA genes until an inducer reduces its affinity.

1) Relevant knowledge points and concepts
- The lac repressor (product of lacI) is a DNA-binding protein that specifically recognizes operator DNA sequences (O1, O2, O3) in the lac operon.  
- Binding is sequence-specific (base-pair contacts), often involving a helix–turn–helix motif.  
- The repressor functions as a tetramer and can bind two operators simultaneously, producing DNA looping that enhances repression.  
- Inducers (allolactose or IPTG) bind the repressor and cause an allosteric change that lowers operator affinity; repressor can bind operator in the absence of inducer.  
- CAP (CRP) is a separate positive regulator that binds cAMP and DNA near the promoter to enhance RNA polymerase binding when glucose is low; CAP is not required for repressor binding.  
- lacI is typically expressed from its own promoter and is often considered constitutive; autoregulation by the repressor is not the primary mechanism controlling lacI.

2) Step-by-step reasoning
- The repressor’s physical mechanism: specific amino-acid side chains contact specific bases in the operator DNA; this is the basis of sequence specificity.  
- In the absence of inducer, the repressor binds operator sequences (O1 primarily) and prevents transcription. Binding does not require lactose/allolactose; rather, inducer binding causes release.  
- CAP’s role is independent and positive; it does not facilitate repressor binding.  
- There is no standard textbook statement that the lac repressor must bind the lacI promoter to regulate its own synthesis.

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

4) Why the correct answer is correct
- “The lac repressor protein binds its operator in a sequence specific way” matches biochemical and genetic evidence: operator sequences and repressor recognition are specific, and mutating operator bases reduces binding affinity and repression.

5) Why other options are incorrect
- “bind to its operator only when it is bound by lactose”: false — the repressor binds operator in the absence of inducer; lactose/allolactose causes dissociation.  
- “also needs the catabolite activator protein to bind to it”: false — CAP acts separately to stimulate transcription; it is not required for repressor binding.  
- “binds to the LacI promoter to regulate its own synthesis”: false as a general statement — lacI is transcribed from its own promoter and is largely constitutive; autoregulation by repressor is not the defining behavior described.

6) Conclusion
The correct description is that the lac repressor binds its operator in a sequence-specific way, which matches the provided answer.

Which of the following accurately describes the binding of lac repressor to the lac operon?Single choice

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