Progressive segregation of the Escherichia coli chromosome.

We have followed the fate of 14 different loci around the Escherichia coli chromosome in living cells at slow growth rate using a highly efficient labelling system and automated measurements. Loci are segregated as they are replicated, but with a marked delay. Most markers segregate in a smooth temporal progression from origin to terminus. Thus, the overall pattern is one of continuous segregation during replication and is not consistent with recently published models invoking extensive sister chromosome cohesion followed by simultaneous segregation of the bulk of the chromosome. The terminus, and a region immediately clockwise from the origin, are exceptions to the overall pattern and are subjected to a more extensive delay prior to segregation. The origin region and nearby loci are replicated and segregated from the cell centre, later markers from the various positions where they lie in the nucleoid, and the terminus region from the cell centre. Segregation appears to leave one copy of each locus in place, and rapidly transport the other to the other side of the cell centre.