Trp Operon Model
Regulation of Bacterial Gene Transcription
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Trp operon (tryptophan operon) is a well-studied example of a repressible operon found in Escherichia coli (E. coli) and other bacteria.
It controls the synthesis of the amino acid tryptophan and functions through a feedback inhibition mechanism, ensuring that the cell only produces tryptophan when it is not available in sufficient quantities from the environment.
Components of the Trp Operon:
Components of the Trp Operon:
Promoter: The DNA sequence where RNA polymerase binds to initiate transcription of the operon.
Operator: A regulatory DNA sequence where the trp repressor protein can bind to block transcription.
Structural Genes: The Trp operon contains five genes (trpE, trpD, trpC, trpB, and trpA) that encode enzymes necessary for the biosynthesis of tryptophan.
trpR Gene: This regulatory gene (located outside the operon) encodes the Trp repressor protein, which regulates the activity of the operon.
Mechanism of the Trp Operon
Mechanism of the Trp Operon
The trp operon is another interesting example of how bacteria manage their resources wisely.
Unlike the lac operon, which turns ON in response to a sugar (lactose), the trp operon is designed to control the production of an essential amino acid i.e., tryptophan. Its regulation works mainly through negative feedback, meaning the cell makes tryptophan only when it is needed and stops making it when enough is already available.
1. In the Absence of Tryptophan
When tryptophan levels are low, the Trp repressor protein (made by the trpR gene) stays inactive. Because the repressor cannot bind to the operator, RNA polymerase freely binds to the promoter and transcribes the operon’s structural genes.
As a result, the cell produces the enzymes required for tryptophan biosynthesis, ensuring the amino acid is made when it is scarce.
2. In the Presence of Tryptophan
When tryptophan levels are high, some tryptophan molecules act as corepressors. They bind to the Trp repressor protein and activate it. The activated repressor now attaches to the operator, blocking RNA polymerase from transcribing the operon.
This shuts down tryptophan biosynthesis, saving the cell from wasting energy and resources when tryptophan is already abundant.
3. Attenuation: Fine-Tuning the Control
The trp operon doesn’t stop at repression, it has a second layer of control called attenuation. This mechanism depends on the formation of special hairpin loops in the mRNA.
If tryptophan is abundant, the ribosome quickly translates a short leader peptide, which allows the mRNA to form a termination loop. This prematurely halts transcription, preventing the production of the biosynthetic enzymes.
If tryptophan is scarce, the ribosome stalls at the leader peptide (because it needs tryptophan to continue). This stalling leads to the formation of an anti-termination loop, allowing full transcription of the operon.
4. Summary of Regulation
Repression: High tryptophan activates the Trp repressor, which blocks transcription at the operator.
Attenuation: High tryptophan also favors a termination loop in the mRNA, further reducing transcription.
Together, these two mechanisms make the trp operon a classic example of feedback inhibition, allowing bacteria to balance tryptophan production according to its availability and avoid unnecessary energy expenditure.
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