Oxolinic acid-resistant mutants of Bacillus subtilis

Two mutants of Bacillus subtilis have been independently selected for resistance to oxolinic acid, and inhibitor of DNA gyrase. The mutations, designated oxr-1 and oxr-2, 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 gyrA mutation described by other workers. In particular, the oxr 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 gyrA mutant is resistant to oxolinic acid, whereas DNA synthesis in the oxr 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 microgram ml-1) can be used to distinguish them from the wild-type. The oxr-1 and oxr-2 mutations define a new genetic locus and can be used as genetic markers in B. subtilis.