Gene regulation (prokaryotic): Which mechanism describes negative regulation of protein synthesis (transcriptional control) in bacteria?

Introduction / Context:
Negative regulation is a cornerstone of bacterial gene control, exemplified by the lac operon. Understanding how repressors modulate transcription distinguishes transcriptional control from enzymatic regulation and positive control mechanisms.

Given Data / Assumptions:

• Negative regulation reduces or blocks transcription.
• Operons use operators where regulatory proteins bind.
• RNA polymerase promoter binding alone is not “negative regulation.”

Concept / Approach:

In negative control, a repressor binds a specific operator DNA sequence near the promoter, physically blocking RNA polymerase binding or elongation. Inducers or corepressors modulate repressor DNA affinity to switch genes on or off based on cellular conditions.

Step-by-Step Solution:

Verification / Alternative check:

Classic systems (lacI–lacO, trpR–trpO) illustrate repressor occupancy reducing transcription; inducers (allolactose) relieve repression by reducing operator affinity.

Why Other Options Are Wrong:

Allosteric inhibition: regulates enzyme activity, not transcription initiation.

RNA polymerase promoter binding: required for transcription, not repression.

Repressor binding to RNA polymerase: not the canonical mechanism for negative control in textbook operons.

Common Pitfalls:

Conflating transcriptional control with enzyme feedback inhibition, or mixing positive and negative control concepts.

Final Answer:

Binding of a repressor protein to DNA (operator)