Why the trp Operon Is a Model of Repressible Regulation
The trp operon in E. coli is a classic example of a repressible operon, meaning it is normally active but can be turned off when enough of its end product is available. This operon controls the synthesis of tryptophan, an essential amino acid. Because producing amino acids requires significant cellular energy, bacteria need a way to stop production when supplies are sufficient. The trp operon provides a precise, efficient mechanism to regulate this process.
The operon consists of a promoter, operator, leader sequence, and five structural genes (trpE, trpD, trpC, trpB, trpA) responsible for producing enzymes needed to synthesize tryptophan. These genes work together to carry out multi-step biochemical pathways, so coordinating their expression is crucial for metabolic efficiency.
Unlike inducible systems such as the lac operon, the trp operon is on by default. The regulatory gene trpR, located elsewhere in the genome, encodes the trp repressor protein. In its inactive form, this repressor cannot bind to the operator. As a result, RNA polymerase transcribes the operon freely, allowing continuous tryptophan synthesis.
The system changes when tryptophan levels in the cell become high. Tryptophan molecules act as corepressors by binding to the trp repressor protein. This binding alters the repressor’s shape, activating it. The active repressor–tryptophan complex binds to the operator region, blocking RNA polymerase from initiating transcription. As a result, the operon shuts off, preventing unnecessary production of tryptophan.
This feedback mechanism conserves energy by adjusting gene expression based on internal conditions. When tryptophan levels drop again, the corepressor detaches, the repressor inactivates, and transcription resumes. The operon toggles on and off as needed, maintaining stable amino acid levels.
A second regulatory layer, attenuation, fine-tunes expression further. The leader sequence contains a regulatory region that responds to tryptophan availability during translation. When tryptophan is abundant, ribosomes move quickly through the leader peptide sequence, forming a transcription-terminating hairpin in the mRNA. This stops transcription prematurely. When tryptophan is scarce, ribosomes stall, allowing transcription to continue. Together, repression and attenuation give the trp operon precise control.
This dual regulation makes the trp operon a powerful model of how bacteria maintain homeostasis.
FAQs
Why is the trp operon considered repressible?
It is repressible because it is usually on but can be turned off when the cell has enough tryptophan. The end product of the pathway—tryptophan—acts as a corepressor that activates the repressor protein, which then blocks transcription.
What is a corepressor in gene regulation?
A corepressor is a molecule that binds to a repressor protein and activates it. In the trp operon, tryptophan binds to the trp repressor, enabling it to attach to the operator and shut off transcription. Without the corepressor, the repressor remains inactive.
What is attenuation, and how does it regulate the trp operon?
Attenuation is a mechanism that terminates transcription early when tryptophan is abundant. It relies on the speed at which ribosomes translate the leader sequence. When tryptophan is scarce, ribosomes stall, preventing termination and allowing transcription of the full operon. This fine-tunes gene expression beyond simple repression.
Study Smarter with RevisionDojo
RevisionDojo helps IB Biology students understand gene regulation with clear, well-structured explanations. The trp operon can feel complex, but our student-friendly notes break it down into manageable steps. Boost your confidence and revision efficiency with RevisionDojo today.
