INITIATION OF CHROMOSOME-REPLICATION AFTER INDUCTION OF DNAA PROTEIN-SYNTHESIS IN A DNAA(NULL) RNH MUTANT OF ESCHERICHIA-COLI

The kinetics of initiation of chromosome replication after induction o f DnaA protein synthesis was studied in a dnaA(null) rnh mutant of Esc herichia coli. DnaA protein synthesis was induced to different extents using the wild-type dnaA gene controlled by a lac promoter. Initiatio n of chromosome replication from oriC, measured as an increase in orig in to terminus ratio, took place at different times after addition of an inducer dependent on the DnaA protein synthesis rate. The first ini tiations always occurred when DnaA protein had accumulated approximate ly to the average wild-type concentration (24 ng of DnaA protein per m l cells at OD450 = 1.0) At a low DnaA protein accumulation rate one sy nchronous round of replication was obtained after 30 min of induction. The initiation kinetics obtained when DnaA protein accumulated rapidl y was complicated and indicated that other factors might also be invol ved.