Initiation of chromosome replication after induction of DnaA protein synthesis in a dnaA(null) rhn mutant of Escherichia coli

The kinetics of initiation of chromosome replication after induction of DnaA protein synthesis was studied in a dnaA(null) rnh mutant of Escherichia coli. DnaA protein synthesis was induced to different extents using the wild-type dnaA gene controlled by a lac promoter. Initiation of chromosome replication from oriC, measured as an increase in origin to terminus ratio, took place at different times after addition of an inducer dependent on the DnaA protein synthesis rate. The first initiations always occurred when DnaA protein had accumulated approximately to the average wild-type concentration (24 ng of DnaA protein per ml cells at OD₄₅₀= 1.0) At a low DnaA protein accumulation rate one synchronous round of replication was obtained after 30min of induction. The initiation kinetics obtained when DnaA protein accumulated rapidly was complicated and indicated that other factors might also be involved.