25. The Lac I and TrpR repressors bind With their ligands allolactose and tryptophan respectively, resulting in alteration of their DNA binding properties. The following statements are made about the mechanism of LacI and TrpR binding with DNA (operator) and regulation of gene expression jn E.coli.
A. Allolactose binding to LacI leads to its poor binding to the lac operator whereas tryptophan binding to TrpR leads to its better binding to the trp operator.
B Allolactose binding to LacI leads to induction of lac operon, whereas tryptophan binding to TrpR leads to repression of trp operon.
C. Binding of allolactose and tryptophan to Lacl and TrpR respectively, leads to repression of their corresponding operons.
D. Binding of allolactose and tryptophan to LacI and TrpR respectively, leads to activation of their corresponding operons. However, in trp operon regulation, availability of tryptophan also results in attenuation-mediate transcriptional termination leading to an overall effect of trp operon.
Which one of the following options represents a combination of all correct statements ?
(1) A and B only            (2) B and C
(3) c and D                   (4) A, B and D

The lac and trp operons in Escherichia coli are iconic models for understanding gene regulation. Both operons are controlled by repressor proteins—LacI for the lac operon and TrpR for the trp operon—which respond to specific small molecule ligands. However, the way these repressors interact with their ligands and the resulting impact on DNA binding and gene expression are fundamentally different. This article clarifies these mechanisms and identifies which statements about LacI and TrpR regulation are correct.

Mechanisms of LacI and TrpR Regulation

LacI and Allolactose: Inducible Regulation

• LacI is the repressor protein for the lac operon. In the absence of lactose, LacI binds tightly to the operator, blocking transcription of the lac genes.

• When lactose is present, it is converted to allolactose, which binds to LacI. This binding causes an allosteric change, reducing LacI’s affinity for the operator DNA, and the repressor dissociates45.

• As a result, RNA polymerase can access the promoter, and the lac operon is induced (turned ON), allowing the cell to metabolize lactose145.

TrpR and Tryptophan: Repressible Regulation

• TrpR is the repressor for the trp operon. In the absence of tryptophan, TrpR cannot bind the operator, and the operon is transcribed to synthesize tryptophan.

• When tryptophan is abundant, it binds to TrpR as a corepressor, causing a conformational change that increases TrpR’s affinity for the operator36. The TrpR-tryptophan complex binds the operator and blocks transcription of the trp operon36.

• This is a classic repressible system, where the presence of the end product (tryptophan) represses its own synthesis.

Evaluating the Statements

A. Allolactose binding to LacI leads to its poor binding to the lac operator whereas tryptophan binding to TrpR leads to its better binding to the trp operator.

• Correct. Allolactose reduces LacI’s DNA binding, while tryptophan enables TrpR to bind DNA more effectively456.

B. Allolactose binding to LacI leads to induction of lac operon, whereas tryptophan binding to TrpR leads to repression of trp operon.

• Correct. Allolactose induces the lac operon by removing repression, and tryptophan represses the trp operon by enabling TrpR to bind the operator3456.

C. Binding of allolactose and tryptophan to Lacl and TrpR respectively, leads to repression of their corresponding operons.

• Incorrect. Allolactose binding to LacI leads to induction, not repression, of the lac operon.

D. Binding of allolactose and tryptophan to LacI and TrpR respectively, leads to activation of their corresponding operons. However, in trp operon regulation, availability of tryptophan also results in attenuation-mediated transcriptional termination leading to an overall effect of trp operon.

• Partially incorrect. Allolactose binding leads to lac operon activation, but tryptophan binding to TrpR leads to repression, not activation, of the trp operon. The statement about attenuation is correct, but the main assertion is not.

Correct Combination

The correct combination is:

(1) A and B only

Summary Table

Conclusion

LacI and TrpR repressors exemplify two major paradigms of gene regulation in bacteria: inducible and repressible systems. Allolactose binding to LacI results in lac operon induction, while tryptophan binding to TrpR results in trp operon repression. The correct statements reflecting these mechanisms are A and B only.

Keywords: LacI, TrpR, repressor, allolactose, tryptophan, lac operon, trp operon, gene regulation, E. coli, operator binding, induction, repression, attenuation.