Q.79 When the supply of tryptophan is plentiful, the tryptophan operon is repressed because the
(A) repressor protein-corepressor complex is bound at the operator
(B) repressor protein is synthesized in large quantity
(C) repressor-corepressor complex is not formed
(D) repressor becomes inactive and therefore has reduced specificity for the operator
Tryptophan operon repression occurs when the repressor protein-corepressor (tryptophan) complex binds the operator, blocking transcription.
The correct answer is (A).
Repression Mechanism
The trp operon is repressible: low tryptophan allows transcription of biosynthetic genes (trpEDCBA). High tryptophan activates trpR-encoded repressor by binding as corepressor, inducing conformational change for operator binding between promoter and trpE. This prevents RNA polymerase access, plus attenuation fine-tunes via leader peptide translation stalls.
Option Breakdown
-
(A) Repressor-corepressor complex bound at operator: Correct—tryptophan (corepressor) allosterically activates apo-repressor for sequence-specific DNA binding, repressing when plentiful.
-
(B) Repressor synthesized in large quantity: Wrong—trpR constitutively low; regulation via activation, not induction.
-
(C) Repressor-corepressor complex not formed: Opposite—complex formation causes repression; absence derepresses.
-
(D) Repressor inactive, reduced operator specificity: Wrong—tryptophan increases specificity; apo-repressor can’t bind tightly.
Introduction to Tryptophan Operon Repressor Corepressor Complex
Tryptophan operon repressor corepressor complex binding explains bacterial repression under nutrient excess—classic GATE Life Sciences genetics topic. High Trp activates repressor for operator block; low Trp allows synthesis. Q.79 tests corepressor mechanics over misconceptions.
Regulation Steps
-
High Trp: Trp binds repressor → conformational shift → operator binding → no transcription.
-
Low Trp: Apo-repressor inactive → RNA pol transcribes despite attenuation.
-
Dual control: Repression (operator block) + attenuation (leader hairpin).
Why Other Options Fail
| Option | Error |
|---|---|
| (B) Large synthesis | Constitutive trpR, not inducible. |
| (C) No complex | Complex REQUIRED for repression. |
| (D) Inactive repressor | Trp activates, doesn’t inactivate. |
| (A) Complex binds | Exact mechanism. |
GATE Tip
Tryptophan operon repressor corepressor complex = Trp as activator of repressor (negative repressible). Contrast lac (inducible). Master for PYQs: (A).
