Gene regulation in bacteria: In the tryptophan (trp) operon of Escherichia coli, which regulatory mechanism prematurely terminates transcription when intracellular tryptophan is abundant?

Introduction / Context:
Bacteria tune gene expression rapidly in response to metabolite levels. The trp operon provides a classic example where transcription can be curtailed early when tryptophan is plentiful, conserving resources by preventing unnecessary synthesis of tryptophan biosynthetic enzymes.

Given Data / Assumptions:

• Tryptophan abundance increases charging of tRNA^Trp.
• Leader peptide coding region and attenuator sequences lie upstream of the structural genes.
• Coupled transcription and translation occur in bacteria.

Concept / Approach:
Attenuation relies on the interplay between ribosome speed on the leader peptide and formation of alternative RNA secondary structures in the leader transcript. When tryptophan is abundant, the ribosome rapidly translates the leader peptide, favoring an RNA terminator hairpin that causes RNA polymerase to release prematurely. When tryptophan is scarce, ribosome stalling promotes an anti-terminator structure, allowing transcription of the operon to continue.

Step-by-Step Solution:
Identify the condition: high tryptophan and highly charged tRNA^Trp. Link rapid leader translation to formation of a terminator hairpin. Recognize the resulting early termination of transcription. Name this mechanism: attenuation.

Verification / Alternative check:
Mutations that disrupt the leader peptide or attenuator structures abolish regulation, demonstrating that RNA structure and translation speed control transcriptional readthrough in the trp operon.

Why Other Options Are Wrong:

• Activation: Increases transcription; opposite effect.
• Corepression: Refers to repressor-operator binding modulated by corepressor; in trp operon it acts at initiation, not the attenuation step.
• Desiccation: Environmental stress, not a regulatory mechanism.
• Riboswitch cleavage: Not the classical mechanism for trp control.

Common Pitfalls:
Confusing initiation control by the Trp repressor with attenuation; both operate, but attenuation specifically involves premature termination based on translation of the leader.

Final Answer:
Attenuation.