Q.32 Which of the following is (are) INCORRECT in the regulation of the trp operon?
(A) It is an example of a negatively controlled repressible operon.
(B) The amino acid Trp inactivates the repressor.
(C) The amino acid Trp induces the operon.
(D) The repressor binds to the operator in the presence of amino acid Trp.
Incorrect options: (B), (C)
The trp operon in E. coli is a classic example of a repressible operon regulated by negative control and attenuation. Tryptophan acts as a corepressor that activates the repressor to shut off transcription when levels are high.
Option Analysis
Option (A): Correct
This accurately describes the trp operon as a negatively controlled repressible system, where the repressor (TrpR) blocks transcription unless derepressed by low tryptophan.
Option (B): Incorrect
Tryptophan activates the repressor by binding as a corepressor, enabling operator binding rather than inactivating it. The apo-repressor (without Trp) cannot bind effectively.
Option (C): Incorrect
Tryptophan represses the operon by promoting repressor-operator interaction, preventing induction. Induction occurs in operons like lac, not trp.
Option (D): Correct
High tryptophan levels allow the TrpR-Trp complex to bind the operator, blocking RNA polymerase and repressing genes (trpE to trpA).
The trp operon regulation mechanism exemplifies repressible control in bacteria, vital for CSIR NET Life Sciences. Genes (trpEDCBA) synthesize tryptophan only when scarce, avoiding wasteful production. This MCQ highlights common pitfalls in trp operon regulation incorrect statements.
Core Mechanism
Low tryptophan keeps the apo-repressor inactive, allowing transcription. Tryptophan binds TrpR, forming a holorepressor that occupies the operator, halting RNA polymerase. Attenuation provides fine-tuning via leader mRNA hairpins.
Why Options B & C Fail
-
Trp activates (not inactivates) the repressor.
-
Trp represses (not induces) the operon, opposite of lac.
| Aspect | Correct Fact | Common Error |
|---|---|---|
| Trp Role | Corepressor activates repressor | Inactivates (B wrong) |
| High Trp Effect | Represses transcription | Induces (C wrong) |
| Repressor Binding | With Trp at operator | Without Trp (opposite) |
Exam Relevance
For competitive exams, distinguish repressible (trp: product off) from inducible (lac: substrate on). Trp operon regulation incorrect statements like B/C test corepressor logic.
