When tryptophan binds to trp repressor, this causes trp repressor to __________ to the trp operator and ________ transcription.

Question

When tryptophan binds to trp repressor, this causes trp repressor to __________ to the trp operator and ________ transcription.

BlackTom AI · Accepted Answer

Tryptophan acts as a corepressor: when it binds the trp repressor protein, the repressor undergoes an allosteric change that enables it to bind the trp operator and block transcription. Key concepts: operons in bacteria, repressors vs activators, and the role of small molecules as inducers or corepressors. The trp operon is a negatively regulated operon that controls genes for tryptophan biosynthesis in E. coli.

Step-by-step reasoning:
1. In the absence of tryptophan the trp repressor protein is produced in an inactive conformation and cannot bind the operator DNA. RNA polymerase therefore can access the promoter and transcribe the operon, producing enzymes for tryptophan biosynthesis.
2. When cellular tryptophan concentration rises, tryptophan molecules bind to the trp repressor. This ligand binding is allosteric: the repressor changes shape upon binding tryptophan.
3. The conformational change converts the repressor into its DNA-binding form. The activated repressor now specifically binds the operator sequence adjacent to the promoter.
4. Operator-bound repressor sterically blocks RNA polymerase binding or progression at the promoter, preventing transcription initiation of the trp operon.
5. This creates a negative feedback loop: high tryptophan → repressor binds → operator is occupied → transcription inhibited → less synthesis of tryptophan-producing enzymes.

Why the correct answer is correct: The mechanism is exactly “bind” (the repressor binds the operator after binding tryptophan) and “inhibits” (binding of the repressor to the operator prevents transcription). The molecule tryptophan functions as a corepressor that activates the repressor’s DNA-binding ability, causing repression (inhibition) of transcription.

Why other options are incorrect:
- “not bind, inhibits”: if the repressor does not bind the operator, it cannot inhibit transcription.
- “bind, activates”: a repressor binding the operator does not activate transcription; it blocks transcription.
- “not bind, activates”: while lack of repressor binding is associated with activation, this option contradicts the premise that tryptophan has bound the repressor.

Final confirmation: bind, inhibits.

When tryptophan binds to trp repressor, this causes trp repressor to __ to the trp operator and transcription.单项选择题

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When tryptophan binds to trp repressor, this causes trp repressor to __________ to the trp operator and ________ transcription.单项选择题