Chromosome segregation in bacteria does not proceed via a mechanism akin to mitosis. Rather, the process appears to resemble sister chromatid formation in eucaryotic cells; a mechanism that is poorly understood in any organism. Our studies, along with those of other laboratories, are starting to provide a detailed picture of the process in Escherichia coli. The essence of this is that the cellular replication machinery is tethered at the cell center, and the DNA passes through it during replication. The newly replicated DNA is directed away from the replication forks toward each cell pole. There, the DNA is refolded and condensed into two separate chromosome masses. We have made progress in elucidating two key activities in the process. First, the SeqA protein, which we discovered by its ability to bind to hemimethylated DNA, appears to be involved in the orderly direction of the newly synthesized DNA away from the replication forks and toward the cell poles. DNA emerging from the forks is uniquely marked by having hemimethylated GATC sequences. These bind SeqA in a highly cooperative fashion, resulting in a tract of bound protein which tracks the forks. These tracts, by interacting with the cell membrane, appear to direct the DNA such that two new chromosomes can form as separate masses. Second, we have found that the MukB, MukE and MukF proteins form an analog of eucaryotic condensins. They act to re-fold and condense the newly replicated DNA into nascent chromosomes in areas of the cell adjacent to the cell poles. This action serves both to organize the chromosomes and to reel in the newly replicated DNA to facilitate its segregation.
