Stable co-existence of separate replicons in Escherichia coli is dependent on once-per-cell-cycle initiation.

DNA replication in most organisms is regulated such that all chromosomes are replicated once, and only once, per cell cycle. In rapidly growing Escherichia coli, replication of eight identical chromosomes is initiated essentially simultanously, each from the same origin, oriC. Plasmid-borne oriC sequences (minichromosomes) are also initiated in synchrony with the eight chromosomal origins. We demonstrate that specific inactivation of newly formed, hemimethylated origins (sequestration) was required for the stable co-existence of oriC-dependent replicons. Cells in which initiations were not confined to a short interval in the cell cycle (carrying mutations in sequestration or initiation genes or expressing excess initiator protein) could not support stable co-existence of several oriC-dependent replicons. The results show that such stable co-existence of oriC-dependent replicons is dependent on both a period of sequestration that is longer than the initiation interval and a reduction of the initiation potential during the sequestration period. These regulatory requirements are the same as those required to confine initiation of each replicon to once, and only once, per cell cycle.