Introduction

The fusion of regulatory sequences from one operon to the structural genes of another is a powerful tool in molecular biology for dissecting gene regulation. In this scenario, the E. coli trp operon, which encodes enzymes for tryptophan biosynthesis, is placed under the control of the lac operator. This means the expression of tryptophan synthase is now governed by the regulatory logic of the lac operon, rather than the native trp system.

The lac Operator: How It Regulates Gene Expression

The lac operon is a classic inducible system controlled by two key sugars:

• Lactose: Acts as an inducer by binding to the lac repressor, causing it to release from the operator and allowing transcription.

• Glucose: When present, it suppresses cAMP levels, reducing the activity of the cAMP-CRP complex, which is necessary for maximal transcription of the lac operon.

Maximal induction of genes under lac operator control occurs when:

• Lactose is present (to inactivate the repressor)

• Glucose is absent (to allow high cAMP and activation by the cAMP-CRP complex)

The trp-lac Chimeric Operon: Regulatory Consequences

In the chimeric strain, the trp operon’s structural genes (including tryptophan synthase) are now expressed according to the lac operator’s rules. The presence or absence of tryptophan is irrelevant; what matters is the availability of lactose and glucose.

Condition for Induction of Tryptophan Synthase

Let’s analyze the options:

1. Only when both lactose and glucose are absent:

   • No lactose: repressor remains bound, operon OFF.

   • No glucose: high cAMP, but irrelevant if operon is OFF.

2. Only when both lactose and glucose are present:

   • Lactose: repressor is inactivated.

   • Glucose: low cAMP, cAMP-CRP complex inactive, operon not fully induced.

3. Only when lactose is absent and glucose is present:

   • No lactose: repressor bound, operon OFF.

4. Only when lactose is present and glucose is absent:

   • Lactose: repressor is inactivated, operon can be ON.

   • Glucose absent: high cAMP, cAMP-CRP complex active, maximal transcription.

Thus, tryptophan synthase will be induced only when lactose is present and glucose is absent5689.

Correct Answer

(4) Only when lactose is present and glucose is absent

Molecular Logic

• Lactose present: Inactivates the lac repressor, allowing RNA polymerase to access the promoter.

• Glucose absent: Increases cAMP, which binds to CRP (cAMP receptor protein), forming a complex that enhances RNA polymerase binding and transcription.

• Combined effect: Maximal induction of the genes downstream of the lac operator, in this case, the trp operon’s structural genes, including tryptophan synthase.

Summary Table

Conclusion

In a strain of E. coli where the trp operon is under the regulatory control of the lac operator, tryptophan synthase will be induced only when lactose is present and glucose is absent. This reflects the classic regulatory logic of the lac operon, now governing tryptophan biosynthesis in the engineered strain.

Keywords: trp operon, lac operator, tryptophan synthase, induction, lactose, glucose, chimeric operon, gene regulation, E. coli, cAMP-CRP complex, molecular biology