Lac operon control—analogy for repressor–DNA binding The binding of the lac repressor to the operator DNA in Escherichia coli is most analogous to which class of enzyme regulation?

Introduction / Context:
Classical bacterial gene regulation is often compared to enzyme regulation. In the lac operon, the lac repressor binds the operator sequence to block RNA polymerase from initiating transcription, providing a clean analogy to enzyme inhibition.

Given Data / Assumptions:

• Lac repressor binds operator DNA adjacent to the promoter.
• Operator occupancy prevents productive binding/initiation by RNA polymerase.
• Inducer (allolactose or IPTG) reduces repressor–operator affinity.

Concept / Approach:
Competitive enzyme inhibition occurs when an inhibitor competes with the substrate for the active site, lowering apparent affinity but not maximal catalytic capacity when substrate is saturating. In the lac system, repressor and RNA polymerase compete functionally for productive occupancy of the promoter–operator region; repressor occupancy precludes polymerase initiation much as a competitive inhibitor precludes substrate binding.

Step-by-Step Solution:
Define the roles: RNA polymerase is analogous to the enzyme; promoter/operator DNA occupancy is the “active site.”Repressor blocks polymerase access—functional competition for the same DNA region.Therefore, the best analogy is competitive inhibition.

Verification / Alternative check:
Induction increases the fraction of time the operator is unoccupied, analogous to raising substrate concentration to overcome a competitive inhibitor and allow catalysis (transcription initiation) to proceed.

Why Other Options Are Wrong:

• Mixed/uncompetitive: These require binding to enzyme–substrate complexes or distinct sites with complex kinetic effects not matching operator occlusion.
• Allosteric effects: The repressor itself is allosterically regulated by inducer, but the question is about repressor binding to DNA, which acts as competitive occlusion of polymerase.
• Irreversible inhibition: Repressor binding is reversible.

Common Pitfalls:
Confusing the allosteric regulation of the repressor protein (by inducer) with the competitive occlusion mechanism at the DNA level.

Final Answer:
Competitive inhibition of an enzyme.