Q27. In tryptophan operon, tryptophan acts as
Correct Answer: (C) Co-repressor
Tryptophan serves as a co-repressor in the trp operon, binding to the inactive repressor protein to activate it and block transcription when levels are high.
Option Analysis
(A) Repressor
The repressor is the trpR gene product, a protein always present but inactive alone; it requires tryptophan to bind the operator. Tryptophan itself is not the repressor protein.
(B) Activator
Activators promote transcription, as in catabolite activator protein for lac operon; trp is repressible, so tryptophan inhibits, not activates.
(C) Co-repressor
Tryptophan binds the apo-repressor, inducing a conformational change for operator binding, halting enzyme synthesis for its own production. This is the precise role.
(D) Co-activator
Co-activators enhance activator binding; absent here, as tryptophan represses the operon via negative control.
The tryptophan operon exemplifies repressible gene regulation in bacteria like E. coli, where tryptophan acts as co-repressor to fine-tune biosynthesis. This mechanism prevents wasteful enzyme production when the amino acid is abundant, crucial for GATE Life Sciences aspirants studying operons.
Core Mechanism
Genes trpEDCBA encode enzymes converting chorismate to tryptophan. Low tryptophan keeps the trpR-encoded repressor inactive, allowing RNA polymerase to transcribe from the promoter past the operator. High tryptophan binds as co-repressor, activating the repressor-operator complex to block transcription (repression).
Attenuation provides secondary control: high tryptophan speeds ribosome translation of leader peptide, forming a terminator hairpin for early termination.
Comparison: trp vs. lac Operon
| Feature | trp Operon (Repressible) | lac Operon (Inducible) |
|---|---|---|
| End Product | Tryptophan (co-repressor) | Allolactose (inducer) |
| Low Signal | Operon ON | Operon OFF |
| High Signal | Operon OFF (repressed) | Operon ON (derepressed) |
| Regulation Type | Negative repressible | Negative inducible |
This table highlights why tryptophan is co-repressor, not repressor or activator.
Exam Relevance
For GATE Life Sciences PYQ like Q27, recall: repressors need ligands in repressible systems; tryptophan fits co-repressor perfectly. Practice distinguishes it from activators in anabolic pathways.
