A Bidimensional Segregation Mode Maintains Symbiont Chromosome Orientation toward Its Host.

All living organisms require accurate segregation of their genetic material. However, in microbes, chromosome segregation is less understood than replication and cell division, which makes its decipherment a compelling research frontier. Furthermore, it has only been studied in free-living microbes so far. Here, we investigated this fundamental process in a rod-shaped symbiont, Candidatus Thiosymbion oneisti. This gammaproteobacterium divides longitudinally as to form a columnar epithelium ensheathing its nematode host. We hypothesized that uninterrupted host attachment would affect bacterial chromosome dynamics and set out to localize specific chromosomal loci and putative DNA-segregating proteins by fluorescence in situ hybridization and immunostaining, respectively. First, DNA replication origins (ori) number per cell demonstrated symbiont monoploidy. Second, we showed that sister ori segregate diagonally prior to septation onset. Moreover, the localization pattern of the centromere-binding protein ParB recapitulates that of ori, and consistently, we showed recombinant ParB to specifically bind an ori-proximal site (parS) in vitro. Third, chromosome replication ends prior to cell fission, and as the poles start to invaginate, termination of replication (ter) sites localize medially, at the leading edges of the growing septum. They then migrate to midcell, concomitantly with septation progression and until this is completed. In conclusion, we propose that symbiont ParB might drive chromosome segregation along the short axis and that tethering of sister ter regions to the growing septum mediates their migration along the long axis. Crucially, active bidimensional segregation of the chromosome allows transgenerational maintenance of its configuration, and therefore, it may represent an adaptation to symbiosis. VIDEO ABSTRACT.