Coordinated replication and sequestration of oriC and dnaA are required for maintaining controlled once-per-cell-cycle initiation in Escherichia coli

Escherichia coli cells were constructed in which the dnaA gene was moved to a location opposite oriC on the circular chromosome. In these cells the dnaA gene was replicated with significant delay relative to the origin. Consequently, the period where the newly replicated and hemimethylated oriC was sequestered no longer coincided with the period where the dnaA gene promoter was sequestered. DnaA protein synthesis was therefore expected to continue during origin sequestration. Despite a normal length of the sequestration period in such cells, they had increased origin content and also displayed asynchrony of initiation. This indicated that reinitiation occasionally occurred at some origins within the same cell cycle. The extra initiations took place in spite of a reduction in total DnaA protein concentration to about half of the wild-type level. We propose that this more efficient utilization of DnaA protein results from an increased availability at the end of the origin sequestration period. Therefore, coordinated sequestration of oriC and dnaA is required for maintaining controlled once-per-cell-cycle initiation.