Cell cycle regulation in Caulobacter: location, location, location.

Cellular reproduction in all organisms requires temporal and spatial coordination of crucial events, notably DNA replication, chromosome segregation and cytokinesis. Recent studies on the dimorphic bacterium Caulobacter crescentus (Caulobacter) highlight mechanisms by which positional information is integrated with temporal modes of cell cycle regulation. Caulobacter cell division is inherently asymmetric, yielding progeny with different fates: stalked cells and swarmer cells. Cell type determinants in stalked progeny promote entry into S phase, whereas swarmer progeny remain in G1 phase. Moreover, initiation of DNA replication is allowed only once per cell cycle. This finite window of opportunity is imposed by coordinating spatially constrained proteolysis of CtrA, an inhibitor of DNA replication initiation, with forward progression of the cell cycle. Positional cues are equally important in coordinating movement of the chromosome with cell division site selection in Caulobacter. The chromosome is specifically and dynamically localized over the course of the cell cycle. As the duplicated chromosomes are partitioned, factors that restrict assembly of the cell division protein FtsZ associate with a chromosomal locus near the origin, ensuring that the division site is located towards the middle of the cell.