29. A mutant Trp repressor (TrpR^m) that cannot bind tryptophan is isolated. In this mutant
A. Trp operon is constitutively expressed.
B. Trp operon is super-repressed.
C. Trp repressor does not bind to the operator.
D. only Trp leader sequence is transcribed.

Which one of the following options represents all correct statements?
(1) A and C only         (2) B and C only
(3) A and D only         (4) A only

Introduction

The trp operon in Escherichia coli is a classic example of a repressible operon, regulated by the availability of tryptophan. The trp repressor (TrpR) protein requires tryptophan as a co-repressor to bind the operator and shut off transcription of the operon. But what happens if a mutant Trp repressor (TrpRm) cannot bind tryptophan at all? This scenario provides valuable insights into the molecular mechanisms of gene regulation and feedback control in bacteria.

Mechanism of trp Operon Regulation

• Wild-type TrpR: In the presence of tryptophan, the amino acid binds to the Trp repressor, enabling it to attach to the operator and repress the operon. In the absence of tryptophan, the repressor cannot bind the operator, so the operon is ON and tryptophan biosynthetic genes are expressed.

• Mutant TrpRm (cannot bind tryptophan): The repressor never becomes active, regardless of tryptophan levels. As a result, it cannot bind to the operator, and the operon remains ON (constitutively expressed) at all times.

Evaluating the Statements

Let’s analyze each statement about TrpRm:

A. Trp operon is constitutively expressed.

• Correct. Since the mutant repressor cannot bind tryptophan, it cannot repress the operon. This leads to continuous (constitutive) expression of the trp operon, regardless of tryptophan availability1358.

B. Trp operon is super-repressed.

• Incorrect. Super-repression would require the repressor to be permanently active, but this mutant cannot repress at all.

C. Trp repressor does not bind to the operator.

• Correct. Without tryptophan binding, the mutant repressor cannot undergo the conformational change needed to bind the operator DNA and block transcription1358.

D. Only Trp leader sequence is transcribed.

• Incorrect. If the repressor cannot bind, the entire operon (not just the leader) will be transcribed.

Correct Combination

The statements that are correct are:

• A. Trp operon is constitutively expressed.

• C. Trp repressor does not bind to the operator.

Therefore, the correct answer is:

(1) A and C only

Biological Significance

A mutant Trp repressor incapable of binding tryptophan mimics a permanent “tryptophan starvation” state for the cell. The operon is always ON, leading to the continuous synthesis of tryptophan biosynthetic enzymes. This demonstrates the importance of allosteric regulation in controlling gene expression and maintaining cellular homeostasis.

Summary Table

Conclusion

When the Trp repressor cannot bind tryptophan due to a mutation, it fails to repress the trp operon. As a result, the operon is constitutively expressed, and the mutant repressor cannot bind the operator. This scenario highlights the critical role of co-repressors in gene regulation.

Keywords: trp operon, Trp repressor, tryptophan, constitutive expression, operator binding, gene regulation, E. coli, mutant repressor, biosynthetic pathway, feedback control