Q.83 Nalidixic acid inhibits gyrase activity. Resistance to this antibiotic arises mainly due to
(A) nonsense mutation in the gyrase gene (B) deletion mutation in the gyrase gene
(C) missense mutation in the gyrase gene (D) degradation of the gyrase gene product
The correct answer is (C) missense mutation in the gyrase gene. Nalidixic acid resistance primarily arises from single amino acid substitutions in the gyrase subunits (GyrA/GyrB), reducing drug binding while preserving enzyme function.
Nalidixic acid targets DNA gyrase (topoisomerase II), inhibiting supercoiling essential for replication. Resistance mutations alter the quinolone-binding pocket. GATE Life Sciences tests this molecular microbiology concept.
Option Analysis
(A) Nonsense mutation in the gyrase gene
Nonsense mutations create premature stop codons, producing truncated, non-functional proteins. This would inactivate gyrase, causing lethality (filamentation, growth defects), not resistance.
(SerB) Deletion mutation in the gyrase gene
Deletions remove DNA segments, typically disrupting protein structure and abolishing function. Gyrase null mutants are inviable; deletions don’t confer resistance by altering binding affinity.
(C) Missense mutation in the gyrase gene
Missense mutations substitute one amino acid (e.g., Ser83→Leu in GyrA), altering the quinolone-binding site. This reduces nalidixic acid affinity while maintaining supercoiling activity—classic resistance mechanism (MIC increases 100-1000x).
(D) Degradation of the gyrase gene product
Gyrase protein degradation would eliminate enzyme activity, preventing replication (bacteriostatic/lethal). Resistance requires functional gyrase with reduced drug sensitivity, not degradation.
Introduction
Nalidixic acid resistance gyrase activity questions test antibiotic mechanisms in GATE Life Sciences. Resistance mainly occurs via missense mutation in the gyrase gene, altering drug binding in E. coli DNA gyrase (GyrA subunit).
DNA Gyrase Inhibition Mechanism
Nalidixic acid stabilizes gyrase-DNA cleavage complex, blocking religation and causing double-strand breaks. Targets GyrA quinolone resistance-determining region (QRDR: codons 67-106). Key mutation: Ser83→Leu/Ile decreases H-bonding.
Resistance Mutation Types
| Mutation Type | Effect on Gyrase | Resistance Outcome |
|---|---|---|
| Missense | Single AA change in binding pocket | Primary resistance mechanism |
| Nonsense | Truncated/loss-of-function | Lethal (no replication) |
| Deletion | Frame shift/protein loss | Inviable mutant |
| Degradation | Enzyme elimination | Growth inhibition |
Clinical & Exam Relevance
First quinolone resistance mutations (1960s) established missense mechanism. Modern fluoroquinolones face similar QRDR mutations. Remember: point mutations (missense) > structural changes for target-based resistance.
GATE PYQ Strategy
Eliminate lethal options (A,B,D). Missense preserves function + reduces affinity = viable resistance. Cross-reference with plasmid-mediated resistance (qnr genes) for advanced questions.
