Inhibitors of DNA synthesis cause excessive DNA synthesis in dnaA mutants of Escherichia coli K12.

The relative rate of net DNA synthesis was stimulated when cells of dnaA mutants of Escherichia coli K12 were grown in the presence of low concentrations of DNA synthesis inhibitors. This led to a supernormal DNA/cell mass ratio. The excessive DNA was similar to the normal chromosomal DNA in size and stability in vivo. However, the cells did not divide but turned into long filaments. Excessive DNA synthesis in the presence of inhibitors of DNA synthesis was observed in the cultures of two independent dnaA mutants of E. coli, but dnaB and dnaC mutants behaved like the wild type in this respect.