Fig. 1. The Caulobacter cell cycle. (A) Each Caulobacter cell division yields a swarmer cell and a stalked cell. Upon differentiating into a stalked cell, the swarmer cell sheds its flagellum, builds a stalk and initiates DNA replication (the chromosome is depicted as a circular black line and as a structure during replication). Just as DNA replication and segregation are concluding, the predivisional cell begins to constrict at the nascent division site. A flagellum is constructed at the pole opposite the stalk, and the completion of cytokinesis generates a new stalked cell and a new swarmer cell. (B) The forward progression of the cell cycle is driven by three master regulators: CtrA, DnaA and GcrA. The levels of each protein oscillate in time over the course of the cell cycle, as indicated graphically, and they successively regulate the transcription of 200 genes.