15. Lac repressor inhibits expression of genes lac-operon whereas purine biosynthesis is repressed by the Pur repressor. The two sequences and have Similar three dimensional The gene regulatory properties of these proteins differ in relation to
(A) binding Of small molecules to the repressor.
(B) presence of recognition Sites on the genome
(C) oligomeric nature of the repressor.
(D) DNA binding property.
The correct statements s
(1) A and B      (2) A, B and C
(3) A and C       (4) B,C and D

 

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The lac repressor and the purine (Pur) repressor are two well-studied proteins in bacterial gene regulation. Both play crucial roles in controlling metabolic pathways—lac repressor for lactose metabolism and Pur repressor for purine biosynthesis. Despite their similar three-dimensional structures and evolutionary origins, these repressors exhibit distinct gene regulatory properties. Understanding these differences is essential for grasping the complexity of operon regulation and allosteric control in bacteria.

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Structural Similarity, Functional Diversity

Both the lac and pur repressors belong to the LacI/GalR family of DNA-binding proteins. They share a highly conserved helix-turn-helix motif in their DNA-binding domains and similar effector binding/oligomerization domains. However, their regulatory mechanisms and responses to cellular signals differ, reflecting their adaptation to distinct metabolic needs.

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Key Differences in Gene Regulatory Properties

A. Binding of Small Molecules to the Repressor

• Lac Repressor:
  The lac repressor is allosterically regulated by inducers such as allolactose or IPTG. Binding of these small molecules decreases the repressor’s affinity for the operator DNA, leading to derepression and transcription of the lac operon genes.

• Pur Repressor:
  The Pur repressor is regulated exclusively by purines (e.g., guanine, hypoxanthine), which act as corepressors. When purines bind to PurR, its affinity for the operator DNA increases, repressing transcription of purine biosynthesis genes.

• Key Point:
  The nature of effector molecules and their regulatory effects are fundamentally different for these repressors12.

B. Presence of Recognition Sites on the Genome

• Lac Repressor:
  Binds to the operator sequence of the lac operon, a specific DNA region upstream of the lac genes.

• Pur Repressor:
  Binds to operator sequences associated with purine biosynthesis genes, which are distinct from those recognized by the lac repressor.

• Key Point:
  The genomic locations and sequences of the operator sites differ, reflecting the distinct metabolic pathways regulated by each repressor15.

C. Oligomeric Nature of the Repressor

• Both repressors form oligomeric complexes (typically tetramers for the lac repressor and dimers/tetramers for PurR), but this property is not a major distinguishing factor in their gene regulatory mechanisms. Their oligomeric states are quite similar due to their shared structural family7.

D. DNA Binding Property

• Both repressors use a highly conserved helix-turn-helix motif for DNA binding, and their DNA binding mechanisms are structurally analogous37. The specificity for operator sequences is determined by subtle differences in amino acid residues and DNA sequence, but the overall mode of DNA interaction is similar.

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Which Features Truly Distinguish Their Regulatory Roles?

Based on the molecular evidence:

• The binding of small molecules (A) and the presence of recognition sites on the genome (B) are the principal differences in gene regulatory properties between the lac and pur repressors.

• Their oligomeric nature (C) and DNA binding property (D) are largely conserved and do not fundamentally distinguish their regulatory mechanisms.

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Correct Answer

(1) A and B

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Biological Significance

The differences in small molecule binding and operator recognition enable bacteria to finely tune gene expression in response to diverse metabolic cues. The lac repressor’s response to lactose availability ensures efficient lactose metabolism, while the Pur repressor’s sensitivity to purine levels maintains nucleotide balance. These adaptive strategies highlight the evolutionary flexibility of regulatory proteins, even among those with highly similar structures.

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Summary Table: Comparison of Lac and Pur Repressors

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Conclusion

While the lac and pur repressors are structurally similar, their gene regulatory properties differ primarily in the types of small molecules that modulate their activity and the specific DNA recognition sites they target in the genome. These differences are central to their distinct roles in bacterial metabolism and gene regulation.

Keywords: lac repressor, purine repressor, gene regulation, E. coli, small molecule binding, operator recognition, DNA binding, oligomeric structure, LacI/GalR family, allosteric regulation, operon, transcriptional control, molecular biology.