Figure 1: E. coli cell just prior to cell division with classic terminus-in / origin-out configuration. Origin (blue) and terminus (red) loci were visualized by FISH, and the nucleoid (grey) by DAPI staining.
Figure 2: Progression of nucleoid, origin and terminus FISH, and DnaX-GFP localization during the first 30 minutes of the cell cycle (cell birth through initiation of DNA replication).
All cells are faced with two very basic problems each cell cycle: (i) they must coordinate DNA replication with cell division such that replication occurs both in a timely manner and only once per cell cycle, and (ii) they must precisely segregate replicated (sister) chromosomes into two daughter cells. Recent work of David Bates and Nancy Kleckner has addressed these problems in the model organism, E. coli (Bates and Kleckner, 2005). Using a novel “baby cell machine”, they have examined chromosome and replisome dynamics in synchronously growing cells by a mixture of cytological and molecular methods. One of the chief discoveries was that replicated sister chromosomes are aligned together (cohesion) for a significant amount of time after they are synthesized and then segregate to opposite sides of the cell in a single global transition. This mode of segregation has strong parallels in eukaryotic cells, and the authors propose that DNA segregation in E. coli may represent a primordial form of modern mitosis, upon which microtubule-based processes were later added.
Another finding is that the chromosome experiences a very specific and predictable change in behavior during the final stages of cell division. After the end of DNA replication up to the time of cell division, sister chromosomes remain relatively static or “frozen” with a characteristic terminus-in / origin-out configuration. Thus, sister chromosomes are oriented tail-to-tail (the terminus being the last part to replicate, and origins being the first) for the later part of the cell cycle. The chromosome masses, or nucleoids, remain in close contact with the midcell division plane (septum) (Figure 1). Concurrent with cell division, however, the nucleoid moves dramatically to the middle of each daughter cell (Figure 2). Origin and terminus loci move in line with the nucleoid, and immediately their positions become highly dynamic, or “unfrozen”. The origin then moves to a fixed assembly of replication proteins (replisome) at midcell and DNA replication is initiated. The authors hypothesize that replication initiation in the next round is triggered or “licensed” by the completion of cell division via these changes in the nucleoid. Such a system ensures a 1:1 relationship between replication and division.
Authors David Bates and Nancy Kleckner