40. Transcriptional regulation of trp operon
by tryptophan involves binding of tryptophan
to
(1) the repressor protein and inhibition of
transcription by its interaction with the
operator region.
(2) RNA polymerase and inhibition of
transcription.
(3) the repressor protein leading to structural
changes and its degradation by proteases.
(4) the repressor protein leading to its
interaction with the sigma subunit and
inhibition of transcription

The regulation of gene expression is fundamental to cellular function, allowing organisms to adapt to their environment by controlling the production of essential molecules. In bacteria such as Escherichia coli, the trp operon is a model system for understanding how gene expression is regulated in response to the availability of a key metabolite—tryptophan. This article explores the molecular mechanism by which tryptophan controls the transcription of the trp operon, the structure and function of the operon, and the broader implications for bacterial metabolism and biotechnology.

Introduction to the trp Operon

The trp operon is a cluster of genes responsible for the biosynthesis of tryptophan, an essential amino acid required for protein synthesis. In E. coli, the trp operon consists of five structural genes (trpE, trpD, trpC, trpB, trpA) that encode enzymes necessary for converting chorismate to tryptophan26. These genes are transcribed as a single mRNA molecule under the control of a common promoter and operator region. The operon is regulated by a repressor protein encoded by the trpR gene, which is located elsewhere on the bacterial chromosome16.

Structure and Function of the trp Operon

The trp operon comprises several key components:

• Promoter: The region where RNA polymerase binds to initiate transcription.

• Operator: A regulatory DNA sequence located between the promoter and the structural genes, serving as the binding site for the trp repressor protein.

• Structural genes: trpE, trpD, trpC, trpB, trpA—these encode the enzymes required for tryptophan biosynthesis.

• Leader sequence: A regulatory region upstream of the first structural gene involved in attenuation, a secondary regulatory mechanism.

The coordinated expression of these genes ensures that tryptophan is synthesized only when needed, conserving cellular resources26.

Transcriptional Regulation by Tryptophan

Transcriptional regulation of the trp operon is a classic example of negative feedback control. When tryptophan is present in sufficient quantities, it acts as a corepressor by binding to the trp repressor protein. This binding induces a conformational change in the repressor, enabling it to bind to the operator region of the trp operon126.

• Tryptophan as a Corepressor: Tryptophan is a small molecule that, when bound to the repressor, activates it. The activated repressor then binds to the operator, physically blocking RNA polymerase from initiating transcription of the structural genes.

• Inhibition of Transcription: By binding to the operator, the repressor prevents RNA polymerase from accessing the promoter, thereby inhibiting the synthesis of the enzymes required for tryptophan biosynthesis126.

Mechanism of Repression

The mechanism of repression is elegantly simple and highly effective:

1. Tryptophan Availability: When tryptophan is abundant in the cell, it binds to the inactive trp repressor.

2. Repressor Activation: The binding of tryptophan changes the shape of the repressor, allowing it to bind to the operator.

3. Transcription Block: The repressor-operator complex blocks RNA polymerase, preventing transcription of the trp operon genes126.

4. Tryptophan Scarcity: When tryptophan is scarce, the repressor remains inactive and cannot bind to the operator. RNA polymerase is free to initiate transcription, and the enzymes for tryptophan biosynthesis are produced.

This regulatory mechanism ensures that the cell does not waste energy producing tryptophan when it is already available in the environment.

Additional Regulatory Mechanisms: Attenuation

In addition to repression by the trp repressor, the trp operon is regulated by attenuation. Attenuation is a fine-tuning mechanism that allows the cell to adjust the expression of the operon based on the intracellular concentration of tryptophan. The leader sequence of the trp mRNA contains regulatory elements that can form alternative secondary structures, leading to premature termination of transcription if tryptophan is abundant125.

Attenuation works in concert with repression to provide robust, multi-layered control over tryptophan biosynthesis.

Answering the Question

The question asks:

Transcriptional regulation of trp operon by tryptophan involves binding of tryptophan to

(1) the repressor protein and inhibition of transcription by its interaction with the operator region.

(2) RNA polymerase and inhibition of transcription.

(3) the repressor protein leading to structural changes and its degradation by proteases.

(4) the repressor protein leading to its interaction with the sigma subunit and inhibition of transcription.

The correct answer is option (1): the repressor protein and inhibition of transcription by its interaction with the operator region126.

• Option (2): Incorrect. Tryptophan does not bind directly to RNA polymerase to inhibit transcription.

• Option (3): Incorrect. Tryptophan binding to the repressor does not lead to its degradation by proteases.

• Option (4): Incorrect. The repressor does not interact with the sigma subunit of RNA polymerase.