Two mutants of have been independently selected for resistance to oxolinic acid, an inhibitor of DNA gyrase. The mutations, designated and map very close to one another but are clearly separated from mutations in the genes for DNA gyrase. Many of the phenotypic properties of the mutants differ from those of a strain containing the mutation described by other workers. In particular, the strains are as sensitive as the wild-type to inhibition by nalidixic acid on solid medium. In addition, experiments with DNA synthesis in toluenized cells show that the enzyme of the mutant is resistant to oxolinic acid, whereas DNA synthesis in the mutants is as sensitive as it is in wild-type preparations. It is concluded that resistance to oxolinic acid is not due to an alteration in the DNA gyrase, but is more probably the result of an impaired uptake of the inhibitor.

Although growth of the mutants on agar plates is inhibited at high concentrations of oxolinic acid, lower concentrations (1-2 μg ml) can be used to distinguish them from the wild-type. The and mutations define a new genetic locus and can be used as genetic markers in


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