TRANSIENT RELAXATION OF PLASMID DNA IN ESCHERICHIA-COLI BY FLUOROQUINOLONES

We examined the influence of fluoroquinolones (norfloxacin, enoxacin, ofloxacin, levofloxacin, and sparfloxacin) on DNA supercoiling of plas mids in Escherichia coli cells by analysis with agarose gel electropho resis in the presence of chloroquine. All the fluoroquinolones tested immediately induced DNA relaxation. The relaxed DNA was re-supercoiled , and the process was sensitive to chloramphenicol, suggesting that ne wly synthesized proteins participate in the reaction. The concentratio ns of fluoroquinolones required for DNA relaxation were much higher th an those required for cell killing. The bactericidal effect of fluoroq uinolones is apparently related to mechanisms other than DNA relaxatio n.