The P1 plasmid is segregated to daughter cells by a 'capture and ejection' mechanism coordinated with Escherichia coli cell division.

The fate of the P1 plasmid of Escherichia coli was followed by time-lapse photomicroscopy. A GFP-ParB fusion marked the plasmid during partition (segregation) to daughter cells at slow growth rate. The process differs from that previously inferred from statistical analysis of fixed cells. A focus of plasmid copies is captured at the cell centre. Immediately before cell division, the copies eject bidirectionally along the long axis of the cell. Cell division traps one or more plasmid copies in each daughter. They are not directed to a prescribed position but are free to move, associate and disassociate. Later, they are captured to the new cell centre to restart the cycle. A null P1 par mutant associates to form a focus, but it is neither captured nor ejected. A dominant negative ParB protein forms a plasmid focus that attaches to the cell centre but never ejects. It remains captive at the centre and blocks host cell division. The cells elongate. Eventually the intact focus is pushed to one side and the cells divide simultaneously in several places at the same time. This suggests that the wild-type plasmid imposes a regulatory node on the host cell cycle, preventing cell division until its own segregation is completed.