Plasmid segregation without partition.

Low-copy number plasmids need a segregation mechanism to assort one half of the plasmid copies to each daughter cell during cell division. This can be achieved directly by partitioning plasmid copies through a mechanism reminiscent of eukaryotic mitosis. Briefly, plasmid copies are paired around a centromere-like site, and then separated toward the daughter cells at cell division. Partition mechanisms are used by a majority of well-studied plasmids. They involve two proteins, a DNA-binding protein and a motor protein, besides the centromeric site. However, some plasmids do not encode typical partition systems, so alternative segregation mechanisms must be considered. For instance, chromosome segregation could provide the driving force for plasmid movement, through a "pilot-fish"-like mechanism. In support of this assumption, we recently demonstrated that plasmid R388 segregation, which does not involve a plasmid-encoded motor protein, requires a single plasmid-encoded DNA-binding protein. Besides, the new segregation system becomes essential when the plasmid encodes conjugation machinery, providing a new understanding of how plasmids integrate conjugative transfer with segregation.