23. In an experimental approach regulatory sequences of trp operon were fused with structural genes of lac operon. Under what condition there would be expression Of β-galactosidase?
(1) Low lactose and glucose
(2) High lactose and glucose
(3) Low tryptophan
(4) High tryptophan

Introduction

In molecular biology, operon fusions are powerful tools for dissecting gene regulation. When regulatory sequences from one operon are fused to the structural genes of another, the expression of downstream genes becomes controlled by the regulatory logic of the donor operon. A classic example is fusing the regulatory region of the trp operon, which is repressible by tryptophan, to the structural genes of the lac operon, such as lacZ encoding β-galactosidase. Understanding when β-galactosidase will be expressed in such a fusion helps illustrate the principles of negative feedback and transcriptional control in bacteria.

The trp Operon: A Repressible System

The trp operon in E. coli is a repressible operon responsible for the biosynthesis of tryptophan. Its key regulatory feature is that it is active (ON) when tryptophan levels are low and repressed (OFF) when tryptophan levels are high. The trp repressor protein binds to the operator only in the presence of tryptophan, blocking transcription when the amino acid is abundant3.

The lac Operon: An Inducible System

The lac operon, in contrast, is typically induced in the presence of lactose and absence of glucose. However, in a fusion where the regulatory region of the trp operon replaces the lac regulatory region, the expression of the lac structural genes (including lacZ for β-galactosidase) will now be governed by the trp operon’s regulatory logic.

The trp-lac Fusion: Regulatory Consequences

In this chimeric construct, the expression of β-galactosidase is no longer responsive to lactose or glucose, but instead to tryptophan levels. Specifically:

• When tryptophan is low: The trp repressor is inactive, the operon is ON, and β-galactosidase is expressed.

• When tryptophan is high: The trp repressor binds to the operator, the operon is OFF, and β-galactosidase is not expressed3.

Answering the Question

Given the options:

1. Low lactose and glucose

2. High lactose and glucose

3. Low tryptophan

4. High tryptophan

The correct answer is:

(3) Low tryptophan

Explanation:
In the trp-lac fusion, β-galactosidase expression is controlled by the trp operon regulatory sequences. Therefore, β-galactosidase will be produced only when tryptophan levels are low, regardless of lactose or glucose concentrations3.

Biological Significance

This fusion experiment elegantly demonstrates how regulatory elements dictate the environmental responsiveness of gene expression. The lacZ gene, normally responsive to lactose, becomes a reporter for tryptophan availability when placed under trp operon control. Such constructs are invaluable for studying gene regulation, feedback inhibition, and the modularity of bacterial promoters and operators.

Summary Table

Conclusion

In a strain where the regulatory sequences of the trp operon are fused to the structural genes of the lac operon, β-galactosidase will be expressed only under low tryptophan conditions. This reflects the regulatory logic of the trp operon, overriding the original lactose-responsive control of the lac operon.

Keywords: trp operon, lac operon, operon fusion, β-galactosidase, gene regulation, tryptophan, E. coli, repressible operon, structural gene, regulatory sequence, molecular biology.