Effects of DNA gyrase inhibitors in Escherichia coli topoisomerase I mutants.

Relaxation of titratable supercoils in bacterial nucleoids was measured following treatment of topA mutants with coumermycin or oxolinic acid, inhibitors of DNA gyrase. Relaxation occurred after treatment of the mutants with either inhibitor. We detected no significant difference in relaxation between topA- and topA+ strains treated with coumermycin. This finding, together with previous observations, supports the idea that relaxation caused by coumermycin probably arises from the relaxing activity of gyrase itself. The source of DNA relaxation caused by oxolinic acid was not identified. Nucleoid supercoiling can be increased by adding oxolinic acid to a strain that carries three topoisomerase mutations: delta topA, gyrB225, and gyrA (Nalr) (S. H. Manes, G. J. Pruss, and K. Drlica, J. Bacteriol. 155:420-423, 1983). We found that this increase in supercoiling requires partial sensitivity to the drug and at the delta topA and gyrA mutations. Full resistance to oxolinic acid in the presence of the delta topA, gyrB225, and gyrA mutations was conferred by an additional mutation that maps at or near gyrB.