Tryptophan operon attenuation: What does the attenuator do when intracellular tryptophan levels are high in E. coli?

Introduction / Context:
The tryptophan (trp) operon of Escherichia coli showcases a classic gene-regulatory strategy called attenuation. It finely tunes transcription according to intracellular amino acid levels, supplementing repressor-operator control for precise feedback regulation.

Given Data / Assumptions:

• High tryptophan means charged tRNA^Trp is abundant.
• The trp operon contains a leader region (trpL) with a short peptide coding sequence and attenuator sequences capable of forming alternative RNA hairpins.
• RNA polymerase initiates at the promoter; ribosomes begin translating the leader as the transcript emerges.

Concept / Approach:
Attenuation couples transcription to translation in bacteria. Depending on tryptophan availability, the leader peptide translation rate alters RNA secondary structure: a terminator hairpin (regions 3–4) forms at high tryptophan and causes premature transcription termination; an anti-terminator hairpin (regions 2–3) forms at low tryptophan and allows continued transcription into structural genes.

Step-by-Step Solution:
At high Trp: ribosome quickly translates leader peptide, covering region 2.Regions 3 and 4 pair to form the terminator hairpin.The terminator structure destabilizes the transcription complex.Transcription stops before structural genes (premature termination).

Verification / Alternative check:
Mutations that prevent 3–4 pairing abolish attenuation; conversely, stabilizing the terminator hairpin enforces early termination regardless of Trp levels.

Why Other Options Are Wrong:

• Allows/propagates transcription: The opposite occurs at high Trp; transcription is curtailed.
• None of the above / activates translation: Attenuation primarily affects transcription elongation, not translation initiation directly.

Common Pitfalls:
Confusing the repressor-operator control (initiation-level) with attenuation (elongation-level, premature termination).

Final Answer:
Attenuator terminates transcription early (premature termination).