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First published online October 10, 2007
doi: 10.1242/10.1242/jcs.005967

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Cell cycle regulation in Caulobacter: location, location, location

Department of Developmental Biology, Beckman Center, Stanford University School of Medicine, 279 Campus Drive, Stanford, CA 94305, USA

View larger version (23K): [in this window] [in a new window]   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.

View larger version (27K): [in this window] [in a new window]   Fig. 2. Spatial mechanisms governing proteolysis of CtrA. (A) The master regulator and inhibitor of DNA replication initiation, CtrA, transiently localizes (red) at the incipient stalked pole at the swarmer (SW) to stalked (ST) cell transition and at the ST pole upon compartmentalization of the cytoplasm in predivisional cells. It localizes there with the protease ClpXP (green), the degradation factor RcdA (orange) and the response regulator CpdR (blue) (in its unphosphorylated state), and is thereby targeted for proteolysis by ClpXP. This relieves the CtrA-mediated block on DNA replication initiation. The cell cycle-regulated phosphorylation state of CpdR is indicated. (B) The hierarchical dependency of ST pole localization for factors necessary for CtrA clearance. Unphosphorylated CpdR at the ST pole is required for polar localization of ClpXP, which is required for the localization of RcdA, which is required for the localization of CtrA.

View larger version (36K): [in this window] [in a new window]   Fig. 3. Chromosome organization and division site specification. (A) Physical arrangement of the 4 megabase pair (Mbp) Caulobacter chromosome over the course of the cell cycle. Regions on the circular chromosome are indicated by color: the origin is shown in yellow, the terminus is shown in red and loci between the two are colored in a gradient according to their position. In swarmer cells, the origin resides near the flagellar/old pole and the terminus resides near the new pole. After initiation of replication, one copy of the origin is segregated to the new pole, and other loci are partitioned as they are replicated. Just before cytokinesis, the terminus regions are separated in the final steps of chromosome segregation. (B) Localization of ParB, MipZ and FtsZ during the cell cycle. ParB associates with the parS sequence adjacent to the origin (see A). MipZ binds to ParB, and both move with one copy of parS to the new pole upon origin segregation. The FtsZ-inhibitory activity of MipZ forces FtsZ from the new pole to the midcell, where it polymerizes into the Z ring that marks the cell division site.