Suppression of temperature-sensitivity of a dnaA46 mutant by excessive DNA supercoiling

We report here that the high-temperature sensitivity of a dnaA46 mutant was suppressed by addition of high concentrations of NaCl into the culture medium. This suppression was also observed with other high-temperature-sensitive dnaA mutants, except dnaA 167 and dnaA508 mutants, which have mutations in the N-terminal region of DnaA protein. Since high concentrations of NaCl in the medium increased negative DNA supercoiling in a dnaA46 mutant, we hypothesized that the increase in DNA supercoiling is involved in the suppression of the temperature sensitivity of the dnaA46 mutant by high concentrations of NaCl. This hypothesis was supported by in vitro and in vivo results as follows. A low DNA replication activity of purified DnaA46 protein at high temperatures was increased in line with an increase in DNA supercoiling of template DNA. The dnaA46 mutant showed higher sensitivity to nalidixic acid, a DNA-relaxing drug, than did the wild-type cells under the conditions of high temperatures and high concentrations of NaCl.