26. In trp operon regulation excess tryptophan act as
(1) Inducer         (2) Activator
(3) Repressor      (4) Co-repressor

 

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Introduction: The trp Operon and Its Regulatory Logic

The trp operon in Escherichia coli is a textbook example of repressible gene regulation. This operon contains a cluster of genes responsible for the biosynthesis of the essential amino acid tryptophan. The operon is regulated so that it is active only when tryptophan is scarce and is turned off when tryptophan is abundant, thus conserving cellular resources and energy135.

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How the trp Operon Works

• When tryptophan is low: The trp operon is “on.” E. coli synthesizes tryptophan by expressing the genes in the operon15.

• When tryptophan is high: The trp operon is “off.” The genes for tryptophan synthesis are repressed to prevent unnecessary production135.

This precise regulation is achieved through a protein called the trp repressor, encoded by the trpR gene, and a regulatory DNA sequence called the operator36.

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Tryptophan as a Co-repressor: The Molecular Mechanism

Excess tryptophan itself is not a repressor, but it plays a critical role in repression by acting as a co-repressor1356. Here’s how the mechanism works:

1. Inactive trp Repressor:
   The trp repressor protein, by itself, cannot bind to the operator region of the trp operon6.

2. Tryptophan Binding:
   When tryptophan levels are high, tryptophan molecules bind to the trp repressor, causing a conformational change that activates the repressor156.

3. Active Repressor-Operator Binding:
   The tryptophan-repressor complex (active repressor) now fits perfectly onto the operator sequence, blocking RNA polymerase from initiating transcription of the trp operon genes36.

4. Result:
   Synthesis of tryptophan biosynthetic enzymes stops, and the cell conserves energy and resources15.

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Why Is Tryptophan Called a Co-repressor?

A co-repressor is a small molecule that partners with a regulatory protein (repressor) to enable it to bind DNA and inhibit gene expression. In the case of the trp operon:

• Tryptophan alone cannot repress the operon.

• The trp repressor alone cannot bind the operator.

• Only the tryptophan-repressor complex can bind the operator and shut down transcription1356.

This is the hallmark of a co-repressor mechanism in gene regulation.

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Attenuation: An Additional Layer of Control

Besides repression, the trp operon also uses attenuation—a mechanism that further reduces gene expression when tryptophan is plentiful. Attenuation works by causing premature termination of transcription in response to high tryptophan levels, adding another layer of negative feedback37.

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Summary Table: Tryptophan’s Role in trp Operon Regulation

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Conclusion: The Correct Answer

Given the options:

1. Inducer

2. Activator

3. Repressor

4. Co-repressor

The correct answer is:
(4) Co-repressor

Excess tryptophan acts as a co-repressor in the trp operon, enabling the trp repressor protein to bind the operator and shut down the expression of tryptophan biosynthetic genes in E. coli1356.

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Keywords: trp operon, tryptophan, co-repressor, gene regulation, E. coli, trp repressor, operator, repression, attenuation, negative feedback, amino acid biosynthesis, molecular biology