DNA dynamics vary according to macrodomain topography in the E. coli chromosome

The organization of the Escherichia coli chromosome has been defined genetically as consisting of four insulated macrodomains and two less constrained regions. Here we have examined the movement of chromosomal loci by tracking fluorescent markers in time-lapse microscopy during a complete cell cycle. Analysing the positioning, the segregation pattern and the motility of markers allowed us to show that the dynamic behaviour of loci belonging to various macrodomains and less constrained regions is radically different. In macrodomains constraints on mobility are apparent whereas in non-structured regions, markers exhibited a greater motility that may explain their ability to interact with flanking macrodomains. Following replication, duplicated markers belonging to macrodomains show a colocalization step and this landmark is not apparent in non-structured regions. Chromosome segregation occurs in three steps: first, the origin-proximal half of the chromosome consisting of the Ori macrodomain and the two non-structured region segregates concomitantly in a short period of time. Second, the Right and Left macrodomains segregate progressively following the genetic map. Third, the Ter macrodomain is rapidly segregated before division, after a significant period of colocalization. Macrodomain territories defined as cellular spaces occupied by the different macrodomains can be identified.