Discussion
Nalidixic acid inhibits gyrase activity. Resistance to this antibiotic arises mainly due to
Answer: C
Resistance arises mainly from a missense mutation in the gyrase gene.
DNA gyrase is essential, so a resistant cell must keep a working enzyme. Nalidixic acid resistance usually comes from single amino-acid substitutions in the GyrA subunit (in the 'quinolone resistance-determining region', for example at serine 83 in E. coli), which lower the drug's binding while leaving the enzyme active.
A nonsense or deletion mutation would destroy the enzyme and kill the cell, and degrading the gyrase protein would do the same, so none of them can be the usual route to resistance.
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