Fig. 10

Model of the two-step chromosome segregation process in H. neptunium. In swarmer cells, the ParB·parS complex interacts with the scaffolding protein PopZ, thus anchoring the ori region at the flagellated pole. At the onset of S-phase, the replisome assembles at ori and the two replication forks start to move independently along the two arms of the chromosome. Soon after the initiation of replication, the two sister ori regions are segregated within the mother cell body, driven by the ParABS chromosome partitioning system. One of the two ori regions is immobilized at the stalk base, potentially through interaction of ParB with a thus-far unknown landmark protein. Once the replication forks have reached the last third of the chromosome, the second segregation step starts and the stalled ori region is translocated through the stalk to the bud compartment. After attachment of the ParB·parS complex to the flagellated bud pole and translocation of the remaining bulk of chromosomal DNA, the cell divides. The resulting stalked cell immediately starts the next replication cycle, whereas the swarmer cell first has to differentiate into a stalked cell to enter S-phase