IDENTIFICATION OF DNA TOPOISOMERASES INVOLVED IN IMMEDIATE AND TRANSIENT DNA RELAXATION INDUCED BY HEAT-SHOCK IN ESCHERICHIA-COLI

The linking number of plasmid DNA in exponentially growing Escherichia coli increases immediately and transiently after heat shock. The purp ose of this study was to search for DNA topoisomerases that catalyze t his relaxation of DNA. Neither introduction of a topA deletion mutatio n nor treatment of cells with DNA gyrase inhibitors affected the DNA r elaxation induced by heat shock. Thus, DNA topoisomerase I and DNA gyr ase are apparently not involved in the process. However, the reaction was inhibited by nalidixic acid or by oxolinic acid in the topA mutant and the reaction was resistant to nalidixic acid in a topA mutant car rying, in addition, the nalA26 mutation. These results are interpreted as indicating that both DNA topoisomerase I and DNA gyrase are involv ed in the DNA relaxation induced by heat shock.