Nalidixic acid resistance: A second genetic character involved in DNA gyrase activity

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Abstract
ATP-dependent DNA supercoiling catalyzed by Escherichia coli DNA gyrase was inhibited by oxolinic acid, a compound similar to but more potent than nalidixic acid and a known inhibitor of DNA replication in E. coli . The supercoiling activity of DNA gyrase purified from nalidixic acid-resistant mutant ( nalA R ) bacteria was resistant to oxolinic acid. Thus, the nalA locus is responsible for a second component needed for DNA gyrase activity in addition to the component determined by the previously described locus for resistance to novobiocin and coumermycin ( cou ). Supercoiling of λ DNA in E. coli cells was likewise inhibited by oxolinic acid, but was resistant in the nalA R mutant. The inhibition by oxolinic acid of colicin E1 plasmid DNA synthesis in a cell-free system was largely relieved by adding resistant DNA gyrase. In the absence of ATP, DNA gyrase preparations relaxed supercoiled DNA; this activity was also inhibited by oxolinic acid, but not by novobiocin. It appears that the oxolinic acid-sensitive component of DNA gyrase is involved in the nicking-closing activity required in the supercoiling reaction. In the presence of oxolinic acid, DNA gyrase forms a complex with DNA, which can be activated by later treatment with sodium dodecyl sulfate and a protease to produce double-strand breaks in the DNA. This process has some similarities to the known properties of relaxation complexes.

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