Decrease in the linking number of plasmid DNA in dnaA mutants of Escherichia coli.

We made use of agarose gel electrophoresis in the presence of chloroquine to examine the linking number of plasmids in temperature-sensitive dnaA mutants, including dnaA5 and dnaA46 mutants. The linking number of DNA prepared from dnaA mutants growing at 37 degrees C was lower than that from wild type cells yet there was no significant difference when cells were grown at 28 degrees C. Complementation analysis with a plasmid containing the wild type dnaA gene and phage P1-mediated transduction confirmed that mutations in the dnaA gene were responsible for the decrease in the linking number of DNA.