Bacterial DNA segregation: its motors and positional control.

A model for DNA segregation in bacteria is proposed which involves not merely growth of the cell membrane and wall, as previously assumed, but also the active movement of one of the two chromosome sister origins by a DNA helicase enzyme and of the chromosome termini and the bulk of the chromosomes by supercoiling tension exerted by DNA gyrase. This provides a unified mechanism for DNA chromosome movement in prosthecate budding bacteria as well as for bacteria that undergo binary fission. The positional control of DNA segregation and the plane of cell division depend, I suggest, on four things: (1) the attachment of the daughter chromosome termini to the cell wall in a position adjacent to the new cell poles at about the time of septation, (2) the displacement of the parental chromosome terminus from this attachment site by the mobile origin, which attaches itself instead to the wall at that point, (3) the movement of the chromosome terminus to a new location in between the daughter origins by the tension of supercoiling, and (4) the determination of the location of the future septum at the position occupied by the chromosome terminus at the time of septal initiation; septum-initiation proteins are postulated to achieve this by binding directly or indirectly to the chromosome terminus. This mechanism automatically ensures ordered DNA segregation in rapidly growing bacteria with more than two sister origins of replication.