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We are studying the underlying principles of cell polarity, asymmetric (stem cell-like) division and cell-cycle control in bacterial cells using the aquatic bacterium Caulobacter crescentus as a model system. At the end of each cell cycle Caulobacter divides unequally into two morphologically dissimilar, but genetically identical, daughter cells: a flagellated dispersal cell known as the `swarmer’ cell and a sessile replicative cell, the ‘stalked’ cell. The two daughter cells are discernible by the presence of different polar appendages. The swarmer cell bears the locomotive engine, the flagellum, and adhesive hair-like filaments, the pili, at the same pole. A cylindrical extension of the cell envelope (the stalk) gives the stalked cell its characteristic appearance. The swarmer cell and the stalked cell differ not only in morphology, but are also physiologically distinct. The chromosome of the swarmer daughter cell is (transiently) inert with respect to DNA replication. The progeny stalked cell, on the other hand, begins to replicate its genome immediately after birth. To proliferate, the non-replicative swarmer cell must first develop into a stalked cell. During this swarmer-to-stalked cell transition the cell pole bearing the pili and the flagellum is remodeled: the flagellum is shed, pili are lost and the growth of the cell envelope is redirected to give rise to the stalk. The differentiating cell also acquires the ability to initiate chromosome replication. Thus, in analogy to eukaryotic cells, the swarmer cell and the stalked cell are comparable to the eukaryotic G1- and S-phase cell, respectively, with the swarmer-to-stalked cell transition corresponding the bacterial the G1-S transition.
Studying the specific mechanisms triggering assembly and disassembly of pili in Caulobacter, also sheds light on central themes such as polarity, asymmetry and cell cycle control. In Caulobacter, pili are only: i) assembled at the cell pole (polarity), ii) found on one of the two daughter cells (asymmetry) and iii) present on G1-phase cells (cell cycle control). We have identified a few players (many remain to be identified) involved in all three processes and are using genetic, cell biological and biochemical techniques to elucidate their functions at the molecular level.
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Selected Publications
Huitema E, Viollier PH. (2006) A myxobacterial S-motility protein dances with poles. Trends Microbiol. 2006 Jun;14(6):247-8. Epub 2006 May 4. [PubMed]
Holtzendorff J, Reinhardt J, Viollier PH. (2006) Cell cycle control by oscillating regulatory proteins in Caulobacter crescentus. Bioessays. 2006 Apr;28(4):355-61. Review. [PubMed]
Huitema E, Pritchard S, Matteson D, Radhakrishnan SK, Viollier PH. (2006) Bacterial birth scar proteins mark future flagellum assembly site. Cell. 2006 Mar 10;124(5):1025-37. [PubMed]
Chen JC, Hottes AK, McAdams HH, McGrath PT, Viollier PH, Shapiro L. (2006) Cytokinesis signals truncation of the PodJ polarity factor by a cell cycle-regulated protease. EMBO J. 2006 Jan 25;25(2):377-86. Epub 2006 Jan 5. [PubMed]
Thanbichler M, Viollier PH, Shapiro L. (2005) The structure and function of the bacterial chromosome. Curr Opin Genet Dev. 2005 Apr;15(2):153-62. Review. [PubMed]
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