The Lac I and Trp R 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 Lac I and Trp R binding with DNA (operator) and regulation of gene expression in 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 the lac operon, whereas tryptophan binding to TrpR leads to repression of the trp operon.
C. Binding of allolactose and tryptophan to LacI 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-mediated transcriptional termination leading to an overall effect of repression of trp operon.

Which one of the following options represents a combination of all correct statements?

Correct option is A

Analysis of the Statements:

1. Statement A: Correct 

   • Allolactose binding to LacI reduces its affinity for the lac operator, making it less likely to bind and thus allowing gene transcription.
   • Tryptophan binding to TrpR increases its affinity for the trp operator, making it more likely to bind and repress transcription.
   • This correctly describes the mechanisms of both lac and trp repressors.

2. Statement B: Correct 

   • The lac operon is an inducible operon, meaning that the presence of allolactose induces transcription by inactivating LacI.
   • The trp operon is a repressible operon, meaning that the presence of tryptophan leads to the repression of transcription.
   • This statement correctly describes the regulation of both operons.

3. Statement C: Incorrect 

   • The statement claims that binding of allolactose to LacI and tryptophan to TrpR leads to repression of their respective operons, which is incorrect.
   • While tryptophan binding to TrpR represses the trp operon, allolactose binding to LacI induces the lac operon by preventing LacI from binding to the operator.
   • Thus, this statement is incorrect because it falsely states that allolactose binding causes repression.

4. Statement D: Incorrect 

   • The statement suggests that binding of allolactose and tryptophan activates both operons, which is partially incorrect.
   • While allolactose activates the lac operon, tryptophan represses the trp operon.
   • Additionally, the attenuation mechanism in the trp operon contributes to repression, but the wording in statement D is misleading because it suggests an overall activation effect.

Final Answer:

• The correct statements are A and B only.
• Option 1 (A and B only) is correct.

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