State of the trp repressor in the presence of intracellular tryptophan: what is it bound to?

Introduction / Context:
The trp operon is a repressible system. Its repressor protein requires tryptophan as a corepressor to bind DNA and block transcription initiation at the promoter.

Given Data / Assumptions:

• Tryptophan acts as a corepressor.
• Repressor–corepressor complex has high affinity for the trp operator.
• Binding to the operator prevents RNA polymerase from initiating transcription.

Concept / Approach:
In high-tryptophan conditions, tryptophan binds the aporepressor, converting it into an active DNA-binding repressor. This complex occupies the operator and reduces transcription of the trp biosynthetic genes.

Step-by-Step Solution:
High Trp → corepressor binds aporepressor.Holo-repressor binds to operator DNA.Transcription initiation is blocked; attenuation provides additional fine control.

Verification / Alternative check:
DNA-binding assays show minimal operator binding without Trp; robust binding occurs upon Trp addition.

Why Other Options Are Wrong:

• Tryp only / DNA only: Both are required for stable operator occupancy under physiological conditions.
• Neither / polymerase binding: Incorrect interactions for repressor function.

Common Pitfalls:
Overlooking that tryptophan is not just a metabolic product but also a regulatory corepressor.

Final Answer:
Bound to both tryptophan (as corepressor) and operator DNA.