34. The tryptophan operon in E. coli is an
example of
(1) inducible operon, controlled by a
repressor protein and attenuation.
(2) repressible operon, controlled by a
repressor protein and attenuation.
(3) inducible operon, controlled by an
activator protein and aporepressor protein.
(4) repressible operon, controlled by a
repressor protein and anti-termination

The regulation of gene expression in bacteria is a masterpiece of biological efficiency, and the tryptophan (trp) operon in Escherichia coli is one of the best-studied examples. The trp operon is not just a simple on-off switch but a sophisticated system that ensures tryptophan, an essential amino acid, is only synthesized when needed. This article explores the trp operon’s structure, its dual regulatory mechanisms, and why it is considered a model of repressible gene regulation.

What Is the trp Operon?

The trp operon is a cluster of five genes—trpE, trpD, trpC, trpB, and trpA—that encode enzymes required for the biosynthesis of tryptophan in E. coli267. These genes are arranged sequentially and transcribed as a single mRNA molecule under the control of a common promoter. This organization allows coordinated expression, ensuring that all the necessary enzymes for tryptophan synthesis are produced together when required.

Repressible vs. Inducible Operons

Before delving deeper into the trp operon, it is essential to distinguish between repressible and inducible operons:

• Inducible Operons: These are typically involved in catabolic pathways, where the presence of a specific molecule (the inducer) turns the operon on. The classic example is the lac operon, which is induced by lactose.

• Repressible Operons: These are involved in anabolic pathways, where the end product of the pathway acts as a corepressor, turning the operon off when it is abundant. The trp operon is a prime example of a repressible operon267.

Regulation by Repressor Protein

The trp operon is negatively regulated by a repressor protein encoded by the trpR gene. When tryptophan is present in the cell, it binds to the trp repressor, causing a conformational change that allows the repressor to bind to the operator region of the operon. This binding blocks RNA polymerase from transcribing the structural genes, effectively shutting down tryptophan biosynthesis267.

Conversely, when tryptophan levels are low, the repressor cannot bind to the operator, allowing RNA polymerase to initiate transcription and produce the enzymes needed to synthesize tryptophan. This mechanism ensures that the cell does not waste energy producing unnecessary enzymes when tryptophan is already available.

Attenuation: A Second Layer of Control

In addition to repression by the trp repressor, the trp operon is regulated by a second mechanism called attenuation. Attenuation is a fine-tuning process that allows the cell to respond to subtle changes in tryptophan availability by controlling the continuation of transcription after it has already started123.

The leader region of the trp operon mRNA contains four segments that can form different hairpin (stem-loop) structures. The formation of these hairpins depends on the speed at which the ribosome translates a short leader peptide encoded by the mRNA. When tryptophan is abundant, the ribosome quickly translates the leader peptide, causing the formation of a terminator hairpin that prematurely terminates transcription. This results in a truncated mRNA and prevents the synthesis of the full set of tryptophan biosynthesis enzymes.

When tryptophan is scarce, the ribosome stalls at tryptophan codons in the leader peptide, allowing the formation of an antiterminator hairpin. This permits RNA polymerase to continue transcription into the structural genes, enabling tryptophan synthesis.

Why Is the trp Operon Repressible?

The trp operon is classified as a repressible operon because the presence of the end product (tryptophan) represses the operon’s expression. This is in contrast to inducible operons, where the presence of a specific molecule induces expression. The trp operon’s repressible nature is perfectly suited for biosynthetic pathways, where it is energetically favorable to turn off the pathway when the end product is already present267.

The Role of the Repressor Protein and Attenuation

The trp operon’s dual regulatory system—repression and attenuation—provides the cell with robust control over tryptophan biosynthesis. The repressor protein acts as a coarse control, turning the operon off when tryptophan is abundant. Attenuation acts as a fine control, allowing the cell to adjust the level of expression in response to minor changes in tryptophan availability123.

This combination ensures that the cell can respond rapidly and efficiently to changes in its environment, conserving energy and resources by only producing tryptophan when it is truly needed.

Comparison with Other Operons

The trp operon is often compared to the lac operon, another well-known example of gene regulation in bacteria. However, the lac operon is inducible and is turned on by the presence of lactose, while the trp operon is repressible and is turned off by the presence of tryptophan. Both operons use repressor proteins, but the trp operon’s additional attenuation mechanism provides an extra layer of control not found in the lac operon27.

Answering the Question

The question asks:
“The tryptophan operon in E. coli is an example of
(1) inducible operon, controlled by a repressor protein and attenuation.
(2) repressible operon, controlled by a repressor protein and attenuation.
(3) inducible operon, controlled by an activator protein and aporepressor protein.
(4) repressible operon, controlled by a repressor protein and anti-termination.”

The correct answer is (2) repressible operon, controlled by a repressor protein and attenuation. The trp operon is repressible, not inducible, and is regulated by both a repressor protein and attenuation, not anti-termination or activator proteins237.

Biological Significance and Applications

Understanding the trp operon’s regulation has broad implications for microbiology, biotechnology, and synthetic biology. By manipulating repressor proteins and attenuation mechanisms, scientists can engineer bacteria to produce desired compounds or respond to specific environmental signals. The principles learned from the trp operon have also inspired the design of synthetic gene circuits for industrial and medical applications.

Conclusion

The trp operon in E. coli is a classic example of a repressible operon, regulated by both a repressor protein and attenuation. This dual regulatory system ensures that tryptophan biosynthesis is tightly controlled, allowing the cell to conserve energy and resources. The trp operon’s elegant design makes it a fundamental model for understanding gene regulation in bacteria and beyond.

In summary, the trp operon is a repressible operon controlled by a repressor protein and attenuation, making option (2) the correct choice. This sophisticated regulatory system highlights the ingenuity of bacterial gene regulation and provides valuable insights for both basic and applied biology.