TRANSIENT DNA RELAXATION IN ESCHERICHIA-COLI INDUCED BY NALIDIXIC-ACID

We report here the transient relaxation of plasmid DNA by inhibitors o f DNA gyrase in Escherichia coli. Relaxation of plasmid DNA in the pre sence of nalidixic acid, an inhibitor of the A subunit of DNA gyrase, lasted less than 60 min, The effect of nalidixic acid was not observed in a nalA26 mutant, a strain resistant to nalidixic acid due to a mut ation in the gyrA gene. Novobiocin, whose target is the B subunit of D NA gyrase, also caused transient DNA relaxation, Resupercoiling of rel axed DNA in the presence of nalidixic acid was inhibited by pretreatme nt of the cells with chloramphenicol. As mutations of both the rpoH an d recA genes did not affect the resupercoiling reaction, this step see ms to require protein synthesis that is independent of both heat shock and SOS responses.