In E.coli, tryptophan switches off the trp operon by

Options

A. inactivating the repressor protein
B. inactivating the gene for the first enzyme in the pathway by feedback inhibition
C. binding to the repressor and increasing the latters affinity for the operator
D. binding to the operator

Answer

### Step-by-Step Solution:

1. **Understanding the TRP Operon**: The TRP operon in E. coli is a set of genes responsible for the biosynthesis of the amino acid tryptophan. It is regulated based on the levels of tryptophan in the environment.

2. **Function of the TRP Repressor**: The TRP operon is controlled by a repressor protein known as the TRP repressor. When tryptophan levels are low, the operon is active, allowing the transcription of genes necessary for tryptophan synthesis.

3. **Role of Tryptophan**: When tryptophan levels are high, tryptophan molecules bind to the TRP repressor. This binding activates the repressor, enabling it to bind to the operator region of the operon.

4. **Mechanism of Switching Off the Operon**: Once the TRP repressor is bound to the operator, it physically blocks RNA polymerase from transcribing the genes of the operon. This effectively switches off the operon, preventing further synthesis of tryptophan.

5. **Conclusion**: Therefore, the correct answer to the question is that tryptophan switches off the TRP operon by binding to the repressor and increasing its affinity for the operator.

### Final Answer:
In E. coli, tryptophan switches off the trp operon by binding to the repressor and increasing the latter's affinity for the operator.

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