# Tryptophan Operon: Regulation and Attenuation Explained

## Introduction to the Tryptophan Operon

The tryptophan operon (often abbreviated as the *trp* operon) is a coordinated cluster of genes in bacteria—most famously studied in *Escherichia coli*—that encodes the enzymes required for the biosynthesis of the amino acid tryptophan. This operon is the paradigm of a **repressible** genetic system: it is actively transcribed when tryptophan is scarce and silenced when tryptophan is abundant. The system employs two distinct but complementary regulatory layers: **negative repression**, which controls the initiation of transcription, and **attenuation**, which controls the premature termination of transcription after initiation has occurred.

Understanding the tryptophan operon is essential for grasping the broader principles of gene regulation in prokaryotes. It demonstrates how a cell integrates metabolic demand with transcriptional output, using both protein-DNA interactions and mRNA structure dynamics. For a foundational review of how operons are organized and defined, see the [Operon Definition](/knowledge/molecular-biology/operon-definition) and the [Operon Concept](/knowledge/molecular-biology/operon-concept).

### What is an operon?

An operon is a functional unit of genomic DNA in prokaryotes that contains a cluster of structural genes transcribed as a single messenger RNA (mRNA) molecule, along with the regulatory sequences that control their transcription. This polycistronic arrangement allows bacteria to coordinately regulate genes whose products participate in the same biochemical pathway. The [Operon Model](/knowledge/molecular-biology/operon-model) was first formalized by François Jacob and Jacques Monod in 1961 based on their studies of the *lac* system, and it remains the central framework for understanding prokaryotic gene expression.

### Genes of the trp operon

The *trp* operon contains five structural genes—*trpE*, *trpD*, *trpC*, *trpB*, and *trpA*—which encode the seven enzymatic activities (some are bifunctional) needed to convert chorismate, a precursor common to several aromatic amino acids, into tryptophan. The operon also includes a promoter (*trpP*), an operator (*trpO*), and a leader sequence (*trpL*) that lies between the operator and the first structural gene. The leader sequence is critical for attenuation, as discussed in detail in later sections.

## Structure of the Tryptophan Operon

The *trp* operon spans approximately 7,000 base pairs on the *E. coli* chromosome. Its architecture is typical of a bacterial biosynthetic operon: regulatory elements at the 5′ end, followed by structural genes arranged in the order of their enzymatic function in the pathway.

### Promoter and operator

The promoter (*trpP*) is the DNA sequence where RNA polymerase (RNAP) binds to initiate transcription. In the *trp* operon, the promoter is relatively weak compared to constitutive promoters, reflecting the fact that tryptophan biosynthesis is energetically expensive and only needed when the amino acid is unavailable from the environment. The promoter contains the canonical −35 and −10 hexamer motifs (TTGACA and TATAAT, respectively, in *E. coli*), though with deviations that reduce binding affinity.

The operator (*trpO*) is a 20-base-pair palindromic sequence that overlaps the promoter region, positioned from approximately −23 to +3 relative to the transcription start site. This overlap is functionally significant: when the Trp repressor protein binds the operator, it physically obstructs RNA polymerase binding, thereby preventing [transcription initiation](/knowledge/molecular-biology/transcription-initiation). The operator is the target of the repressor-operator interaction that constitutes the first layer of regulation.

### Leader peptide and attenuator

Between the operator and *trpE* lies the leader sequence (*trpL*), a 162-nucleotide region that is transcribed but not translated into a functional protein. Instead, the *trpL* mRNA contains two critical features:

1. A short open reading frame (ORF) encoding a 14-amino-acid leader peptide with the sequence **Met-Lys-Ala-Ile-Phe-Val-Leu-Lys-Gly-Trp-Trp-Arg-Thr-Ser**. Notably, this peptide contains two consecutive tryptophan codons (UGG UGG) near its center.
2. Four regions of sequence complementarity (designated 1, 2, 3, and 4) that can form alternative stem-loop structures in the mRNA.

The positions of the tryptophan codons within the leader ORF and the alternative base-pairing possibilities among regions 1–4 are the mechanistic heart of attenuation. The leader also contains a **rho-independent transcription terminator** (a GC-rich hairpin followed by a poly-U tract) that forms when regions 3 and 4 pair, causing RNA polymerase to dissociate and release a truncated 140-nucleotide RNA.

## Negative Repression by the Trp Repressor

The first layer of *trp* operon regulation is a classic negative feedback loop mediated by the **Trp repressor** protein, encoded by the unlinked *trpR* gene. The repressor is constitutively synthesized at low levels and exists as a homodimer in solution.

### Role of tryptophan as co-repressor

The Trp repressor is an **allosteric protein**: it exists in two conformations, and its DNA-binding affinity depends on the occupancy of its ligand-binding site. Tryptophan serves as the **co-repressor**—a small molecule that, when bound to the repressor, increases its affinity for the operator. In the absence of tryptophan, the repressor adopts a conformation with low operator affinity and dissociates from DNA, allowing transcription to proceed. When tryptophan levels rise, two molecules of tryptophan bind to the repressor dimer (one per monomer), inducing a conformational change that repositions the helix-turn-helix DNA-binding motifs into the major groove of the operator sequence.

The binding of the tryptophan-repressor complex to *trpO* is cooperative and highly specific. The equilibrium dissociation constant (Kd) for the repressor-operator interaction is approximately 10⁻⁹ M in the presence of tryptophan, compared to roughly 10⁻⁶ M in its absence—a 1,000-fold difference in affinity. This sharp switch ensures that the operon is effectively fully repressed when intracellular tryptophan exceeds approximately 100 µM.

### Binding affinity and regulation

The operator sequence is a near-perfect palindrome: 5′-ATGTACTAGTAACTAGTACAT-3′ (with symmetry around the central base). Each repressor dimer contacts one half-site, and because the operator overlaps the −10 element of the promoter, bound repressor sterically blocks RNA polymerase binding. This is a key distinction from some other repressible systems where the repressor may block polymerase escape rather than initial binding.

The repression ratio—the ratio of transcription in the absence versus presence of tryptophan—is approximately 70-fold. This is substantial but not absolute; residual transcription continues even under high tryptophan conditions. This "leakiness" is biologically meaningful, as it provides a basal level of biosynthetic enzymes that can respond rapidly if tryptophan becomes limiting. The remaining regulation—attenuation—provides an additional ~8- to 10-fold reduction, bringing the total dynamic range of the system to roughly 500- to 700-fold.

## Attenuation: A Fine-Tuning Mechanism

Attenuation is a regulatory mechanism that operates after transcription has initiated but before RNA polymerase has committed to transcribing the entire operon. It relies on the coupling of [transcription and translation](/knowledge/molecular-biology/transcription-translation) in bacteria—events that occur simultaneously in the cytoplasm because there is no nuclear membrane. The *trpL* leader sequence is the platform for this regulation.

### The leader peptide and tryptophan codons

The leader mRNA is translated by a ribosome immediately after its 5′ end emerges from RNA polymerase. The leader ORF encodes the 14-amino-acid peptide described earlier, which contains two adjacent tryptophan residues. Tryptophan is the rarest amino acid in proteins, and its cognate tRNA (tRNA^Trp) is correspondingly present at low abundance. The rate at which the ribosome translates the two UGG codons is therefore exquisitely sensitive to the concentration of charged tRNA^Trp (tryptophanyl-tRNA).

The key principle is this: **the position of the ribosome on the leader mRNA at any given moment determines which RNA secondary structures can form**, and that in turn determines whether transcription continues or terminates.

### Alternative RNA secondary structures (3-4 vs 2-3)

The four regions of the leader mRNA can pair in two mutually exclusive ways:

- **The 2-3 hairpin** (regions 2 and 3 base-pair) is an **antiterminator** structure. When it forms, region 4 remains single-stranded and cannot form the terminator. Transcription continues into the structural genes.
- **The 3-4 hairpin** (regions 3 and 4 base-pair) is the **terminator** structure. It is a GC-rich stem-loop followed by a run of uracil residues—the classic rho-independent terminator. When it forms, RNA polymerase pauses at the poly-U tract, the RNA-DNA hybrid dissociates, and transcription stops, releasing a 140-nucleotide RNA.

Regions 1 and 2 can also pair (the 1-2 hairpin), but this structure is transient and primarily serves to sequester region 2 when the ribosome is stalled at the tryptophan codons, thereby preventing the 2-3 antiterminator from forming.

The decision between the 2-3 and 3-4 structures is dictated by the ribosome's position, which is in turn dictated by tryptophan availability. This is the essence of attenuation: a kinetic competition between RNA polymerase transcribing the leader and the ribosome translating it.

## The Leader mRNA and Ribosome Stalling

The attenuation mechanism is best understood by walking through the two extreme scenarios: high tryptophan and low tryptophan. In both cases, the ribosome begins translating the leader ORF as soon as the ribosome-binding site (Shine-Dalgarno sequence) and start codon emerge from RNA polymerase.

### High tryptophan conditions

When tryptophan is abundant, charged tRNA^Trp is plentiful, and the ribosome translates the two UGG codons rapidly without pausing. The sequence of events is as follows:

1. RNA polymerase initiates transcription at *trpP* and begins synthesizing the leader mRNA.
2. A ribosome binds to the Shine-Dalgarno sequence on the nascent mRNA and starts translating the leader peptide.
3. The ribosome translates through the two tryptophan codons without delay because charged tRNA^Trp is available.
4. As the ribosome proceeds, it reaches the stop codon (UGA) at the end of the leader ORF, which is located in region 1. The ribosome then releases the completed 14-amino-acid peptide and dissociates from the mRNA.
5. Critically, the ribosome has now physically covered region 2 of the mRNA. Because region 2 is sequestered within the ribosome, it cannot pair with region 3.
6. When RNA polymerase transcribes region 4, region 3 is free to pair with region 4, forming the 3-4 terminator hairpin.
7. The terminator causes RNA polymerase to pause and then dissociate at the poly-U tract. Transcription is prematurely terminated, and the structural genes are not expressed.

This outcome is summarized in the table below.

### Low tryptophan conditions

When tryptophan is scarce, the concentration of charged tRNA^Trp is low, and the ribosome stalls at the tandem UGG codons. The sequence of events diverges:

1. RNA polymerase initiates transcription as before, and a ribosome binds and begins translating the leader ORF.
2. The ribosome translates the early codons (encoding Met-Lys-Ala-Ile-Phe-Val-Leu-Lys-Gly) but stalls when it reaches the two tryptophan codons because uncharged tRNA^Trp cannot be efficiently accommodated in the A site.
3. The stalled ribosome physically occupies region 1 and part of region 2, but it does **not** reach region 2's distal portion. Importantly, the stall position leaves region 2 exposed and available for base-pairing.
4. Because region 1 is sequestered inside the stalled ribosome, the 1-2 hairpin cannot form. Instead, region 2 pairs with region 3, forming the 2-3 antiterminator structure.
5. Region 4 remains single-stranded because region 3 is now engaged with region 2. The 3-4 terminator cannot form.
6. RNA polymerase continues transcription through region 4 and into *trpE*, transcribing the entire operon. The structural genes are expressed, and the biosynthetic enzymes are produced.

The table below summarizes the two conditions.

| Condition | Ribosome position | Pairing outcome | Transcription result |
|-----------|-------------------|-----------------|----------------------|
| High tryptophan | Translates through Trp codons, reaches stop codon, covers region 2 | 3-4 terminator forms | Termination after ~140 nt |
| Low tryptophan | Stalls at Trp codons, region 2 exposed | 2-3 antiterminator forms | Full operon transcribed |
| Intermediate (partial charging) | Slow translation, some pausing | Mixed; probabilistic | Partial readthrough |

The system is not digital but analog: at intermediate tryptophan concentrations, the ribosome stalls for intermediate durations, and the probability of terminator versus antiterminator formation reflects the balance. This allows graded regulation of tryptophan biosynthetic capacity.

## Experimental Evidence and Methods

The two-layer model of *trp* operon regulation emerged from decades of genetic and biochemical investigation. Understanding the experimental basis of these findings is valuable for appreciating how the model was established.

### Reporter assays

The classical approach to studying operon regulation is to fuse the regulatory region of interest to a **reporter gene** whose product is easily assayed. In early studies, the *trp* operon was fused to the *lacZ* gene encoding β-galactosidase. Cells carrying such fusions were grown in media with defined tryptophan concentrations, and β-galactosidase activity was measured spectrophotometrically using the chromogenic substrate ONPG (ortho-nitrophenyl-β-galactoside). These assays quantified the repression ratio and demonstrated that tryptophan addition reduced expression approximately 70-fold.

To separate the contributions of repression and attenuation, researchers constructed strains with mutations in *trpR* (eliminating the repressor) or with deletions of the leader region. A *trpR⁻* strain still showed ~8- to 10-fold regulation due to attenuation alone, while a leader deletion abolished attenuation but retained repression. Combining both mutations eliminated regulation entirely.

### RNA secondary structure analysis

Direct evidence for the alternative hairpin structures came from **RNA structure probing** experiments. Leader RNA was synthesized in vitro and incubated under conditions that favor one structure or the other. Enzymatic probes such as RNase T1 (which cleaves single-stranded RNA after guanosine residues) and RNase V1 (which cleaves double-stranded RNA) were used to map which regions were paired and which were exposed.

In these experiments, RNA transcribed in the presence of high tryptophan (or in the absence of ribosomes) showed cleavage patterns consistent with the 3-4 hairpin. RNA transcribed under low-tryptophan conditions, or in the presence of stalled ribosomes, showed protection of region 1 and exposure of region 4, consistent with the 2-3 antiterminator. These biochemical data complemented the genetic evidence and confirmed the model.

Additional support came from **site-directed mutagenesis** of the leader sequence. Mutations that destabilized the 3-4 hairpin reduced termination, while mutations that destabilized the 2-3 hairpin increased termination. Compensatory mutations that restored base-pairing also restored the wild-type phenotype, demonstrating that the secondary structures, not the primary sequence per se, were the functional elements.

## Comparison with Other Operons

The *trp* operon is often taught alongside the *lac* operon, and the contrast between the two systems is instructive. For a detailed treatment of the *lac* system, see the [Lac Operon](/knowledge/molecular-biology/lac-operon) and the [Lactose Operon](/knowledge/molecular-biology/lactose-operon).

### Inducible vs repressible operons

The *lac* operon is **inducible**: it is normally off and is turned on by the presence of lactose (or its analog IPTG). The *lac* repressor binds the operator in the absence of the inducer and is released when allolactose (the natural inducer) binds it. The *trp* operon is **repressible**: it is normally on and is turned off by the presence of tryptophan. The *trp* repressor binds the operator only when tryptophan is present.

This difference reflects the biological logic: *lac* encodes catabolic enzymes that break down a sugar (lactose) for energy—the cell only needs them when the substrate is available. The *trp* operon encodes anabolic enzymes that build an amino acid—the cell only needs them when the product is absent. Both systems use negative regulation, but the default states are opposite.

Another key difference is that the *lac* operon is also subject to **positive regulation** by the catabolite activator protein (CAP)-cAMP complex, which responds to glucose availability. The *trp* operon lacks a comparable positive control mechanism; its regulation is entirely negative (repression plus attenuation).

### Attenuation in other operons

Attenuation is not unique to the *trp* operon. Many amino acid biosynthetic operons in *E. coli* and *Salmonella* use the same general strategy, with leader peptides enriched in codons for the amino acid being synthesized:

- The **histidine (*his*) operon** has a leader peptide containing seven consecutive histidine codons.
- The **phenylalanine (*phe*) operon** has a leader with multiple phenylalanine codons.
- The **leucine (*leu*) operon** has a leader with multiple leucine codons.
- The **threonine (*thr*) operon** and **isoleucine-valine (*ilv*) operon** use similar mechanisms.

In each case, the leader peptide is rich in the relevant amino acid, and ribosome stalling at those codons determines whether the antiterminator or terminator hairpin forms. The *trp* operon is the best-studied example, but the principle is general.

The [Arabinose Operon](/knowledge/molecular-biology/arabinose-operon) provides another contrast: it is a catabolic system regulated by both positive and negative control through the AraC protein, which acts as an activator in the presence of arabinose and a repressor in its absence. This dual-function regulator is distinct from the simple repressor of the *trp* system.

## Common Pitfalls and Misconceptions

Students frequently encounter several conceptual difficulties when studying the *trp* operon. Identifying these early can prevent confusion on exams.

### Repression vs attenuation

The most common error is conflating repression and attenuation. These are **two separate mechanisms operating at different stages**:

- **Repression** controls **[transcription initiation](/knowledge/molecular-biology/transcription-initiation)**. It is mediated by the Trp repressor protein binding to the operator and blocking RNA polymerase. It responds to tryptophan over a relatively slow timescale (minutes) and provides the coarse, ~70-fold regulation.
- **Attenuation** controls **transcription elongation** (premature termination). It is mediated by RNA secondary structure and ribosome positioning. It responds more rapidly and provides the fine, ~8- to 10-fold regulation.

A useful mnemonic: repression is about **whether** transcription starts; attenuation is about **whether** transcription that has started continues. Both are repressed by high tryptophan, but they operate through entirely different molecular mechanisms.

### Role of the leader peptide

Another common misconception is that the leader peptide itself is a regulatory protein or that it has enzymatic function. It does not. The 14-amino-acid leader peptide is **not the product of the operon** in any functional sense; it is simply a translation sensor. Its only role is to position the ribosome at a location that depends on tryptophan availability. The peptide is rapidly degraded after synthesis and has no biological activity.

Relatedly, students sometimes think that the leader peptide is the tryptophan biosynthetic enzyme. It is not—the enzymes are encoded by *trpE*, *trpD*, *trpC*, *trpB*, and *trpA*. The leader is a regulatory element, not a structural gene.

### Tryptophan as an inducer

Some students, coming from the *lac* operon, assume that tryptophan acts as an inducer. This is incorrect. Tryptophan is a **co-repressor** in the *trp* system. It does not activate transcription; it represses it. The default state of the *trp* operon is "on" (transcription occurs), and tryptophan turns it off. This is the opposite of the *lac* operon, where the default state is "off" and lactose (or IPTG) turns it on.

### The direction of the attenuation logic

A subtle but important point: in the *trp* operon, **high tryptophan causes termination**, and **low tryptophan allows readthrough**. This is counterintuitive to some students who expect that a regulatory mechanism would "sense" low tryptophan and then act to terminate. In fact, the default outcome of transcription is termination (because the 3-4 hairpin is thermodynamically more stable than the 2-3 hairpin). The 2-3 antiterminator only forms when the ribosome stalls, which only happens under low-tryptophan conditions. Thus, attenuation is a **default-off** system that is **activated** by tryptophan starvation.

## Summary and Study Tips

The *trp* operon is regulated by two independent but coordinated mechanisms that respond to intracellular tryptophan concentration.

### Key points to remember

1. The *trp* operon is a **repressible** operon: it is transcribed by default and repressed when tryptophan is abundant.
2. **Repression** (70-fold) is mediated by the Trp repressor protein, which binds tryptophan as a co-repressor and then binds the operator, blocking RNA polymerase.
3. **Attenuation** (8- to 10-fold) is mediated by the leader mRNA, which can form either a 2-3 antiterminator hairpin (allowing readthrough) or a 3-4 terminator hairpin (causing premature termination).
4. The decision between hairpins is made by the ribosome: stalled at tryptophan codons under low tryptophan (antiterminator forms), or translating through and covering region 2 under high tryptophan (terminator forms).
5. The leader peptide is a sensor, not a functional product.
6. The total regulation is multiplicative: ~70 × ~8 = ~560-fold dynamic range.
7. Attenuation is used by many amino acid biosynthetic operons, not just *trp*.

### Practice questions

To test your understanding, try to answer the following without referring to notes:

1. Draw the *trp* operon, labeling the promoter, operator, leader, and structural genes. Indicate where the repressor binds and where RNA polymerase binds.
2. Explain why the 3-4 hairpin causes termination but the 2-3 hairpin does not.
3. Predict the phenotype of a strain with a mutation that eliminates the leader peptide start codon. Would the operon be constitutively expressed or constitutively repressed? (Answer: constitutively expressed, because the ribosome cannot translate the leader, so region 2 is never covered, and the 2-3 antiterminator always forms.)
4. Predict the phenotype of a strain with a mutation that destabilizes the 3-4 hairpin. (Answer: reduced termination; operon is expressed even at high tryptophan.)
5. Compare and contrast the roles of the *lac* repressor and the *trp* repressor, including their default DNA-binding states.

## Frequently Asked Questions

### What is the tryptophan operon?

The tryptophan operon (*trp* operon) is a cluster of five genes in bacteria that encode the enzymes for tryptophan biosynthesis. It includes a promoter, an operator, and a leader sequence that together allow the cell to regulate expression based on tryptophan availability. It is the classic example of a [repressible operon](/knowledge/molecular-biology/repressible-operon).

### How does the tryptophan operon work?

The *trp* operon is regulated by two mechanisms. First, the Trp repressor protein, when bound to tryptophan, binds the operator and blocks RNA polymerase from initiating transcription (repression). Second, the leader mRNA can form alternative secondary structures that either permit continued transcription or cause premature termination (attenuation). Both mechanisms respond to tryptophan levels and together provide ~500-fold regulation.

### What is the function of the tryptophan operon?

The function of the *trp* operon is to produce the enzymes needed to synthesize tryptophan from chorismate. The operon ensures that these enzymes are produced only when tryptophan is scarce, preventing wasteful production when the amino acid is available from the environment or already present in the cell.

### What is attenuation in the tryptophan operon?

Attenuation is a regulatory mechanism that prematurely terminates transcription of the *trp* operon when tryptophan is abundant. It relies on the formation of a terminator hairpin (3-4) in the leader mRNA, which causes RNA polymerase to dissociate. When tryptophan is scarce, a ribosome stalls at tryptophan codons in the leader peptide, allowing an antiterminator hairpin (2-3) to form instead, and transcription continues.

### What is the role of the leader peptide in the trp operon?

The leader peptide is a 14-amino-acid peptide encoded by the *trpL* leader sequence. It contains two consecutive tryptophan codons. Its role is to sense tryptophan availability: when tryptophan is scarce, the ribosome stalls at these codons, which determines which RNA secondary structure forms in the leader mRNA. The peptide itself has no enzymatic or regulatory function beyond this sensing role.

### How is the trp operon different from the lac operon?

The *trp* operon is repressible (normally on, turned off by tryptophan), while the *lac* operon is inducible (normally off, turned on by lactose). The *trp* repressor binds DNA only when tryptophan is present, whereas the *lac* repressor binds DNA in the absence of lactose and is released when lactose is present. Additionally, the *trp* operon uses attenuation as a second regulatory layer, while the *lac* operon uses positive regulation by CAP-cAMP.

### What happens when tryptophan is low in the cell?

When tryptophan is low, charged tRNA^Trp is scarce. The ribosome translating the leader peptide stalls at the two tryptophan codons. This leaves region 2 of the leader mRNA available to pair with region 3, forming the 2-3 antiterminator. The 3-4 terminator cannot form, so RNA polymerase continues transcribing the structural genes, and tryptophan biosynthetic enzymes are produced.

## Key Takeaways

- The *trp* operon is a [repressible operon](/knowledge/molecular-biology/repressible-operon) encoding five enzymes for tryptophan biosynthesis, regulated by both repression and attenuation.
- Repression is mediated by the Trp repressor-tryptophan complex binding the operator and blocking RNA polymerase, providing ~70-fold regulation.
- Attenuation is mediated by alternative RNA secondary structures in the leader mRNA, providing an additional ~8- to 10-fold regulation.
- The leader peptide contains tandem tryptophan codons; ribosome stalling at these codons under low tryptophan allows the 2-3 antiterminator to form.
- Under high tryptophan, the ribosome translates through and covers region 2, allowing the 3-4 terminator to form and terminate transcription.
- The two mechanisms are multiplicative, giving a total dynamic range of ~500- to 700-fold.
- Attenuation is a general strategy used by many amino acid biosynthetic operons, including *his*, *phe*, *leu*, and *thr*.

## Further Reading

- Xie G et al. *Ancient origin of the tryptophan operon and the dynamics of evolutionary change*. Microbiology and [molecular biology](/blog/careers/molecular-biology) reviews : MMBR. 2003. [PubMed 12966138](https://doi.org/10.1128/MMBR.67.3.303-342.2003)
- Imamoto F. *Translation and transcription of the tryptophan operon*. Progress in nucleic acid research and molecular biology. 1973. [PubMed 4573491](https://doi.org/10.1016/s0079-6603(08)60107-5)
- Yanofsky C et al. *The complete [nucleotide sequence](/knowledge/molecular-biology/nucleotide-sequence) of the tryptophan operon of Escherichia coli*. [Nucleic acids research](/blog/news/nucleic-acids-research). 1981. [PubMed 7038627](https://doi.org/10.1093/nar/9.24.6647)
- Hiraga S. *[Correlation of transcription and translation in the tryptophan operon of Escherichia coli (author's transl)]*. Tanpakushitsu kakusan koso. Protein, nucleic acid, enzyme. 1973. [PubMed 4582341](https://pubmed.ncbi.nlm.nih.gov/4582341/)
- Ito K. *[Genetic control of amino acid synthesis system in bacteria--regulation mechanism of tryptophan operon]*. Tanpakushitsu kakusan koso. Protein, nucleic acid, enzyme. 1972. [PubMed 4555599](https://pubmed.ncbi.nlm.nih.gov/4555599/)
- Bommana S et al. *Tryptophan Operon Diversity Reveals Evolutionary Trends among Geographically Disparate Chlamydia trachomatis Ocular and Urogenital Strains Affecting Tryptophan Repressor and Synthase Function*. mBio. 2021. [PubMed 33975934](https://doi.org/10.1128/mBio.00605-21)

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