Caulobacter crescentus

Caulobacter crescentus is a Gram-negative, oligotrophic bacterium widely distributed in fresh water lakes and streams. It plays an important role in the carbon cycle.

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Caulobacter is an important model for study of the regulation of the cell cycle and cellular differentiation. Caulobacter daughter cells are very different from each other. One is a mobile "swarmer" cell that has a flagellum for swimming. The other, called the "stalked" cell has a long tubular stalk structure protruding from one pole that has an adhesive holdfast material on its end, with which the stalked call can adhere to surfaces. Chromosome replication and cell division only occurs in the stalked cells. Swarmer cells differentiate into stalked cells as they mature. Often surviving in nutrient-poor environs, Caulobacter crescentus is a Gram-negative bacterium ubiquitous in fresh water, soil, and sea water. C. crescentus exhibits a dimorphic life cycle that most likely provides an advantage in such competitive environments. The stalk cell can attach to a surface, while the swarmer cell can search for nutrients. The adhesive material of the holdfast has been reported to be one of the strongest natural glues.

Caulobacter cell cycle

Because of its easy manipulation in laboratory, Caulobacter has become a model organism to investigate cell cycle regulation in bacteria. Of Caulobacter's 3767 protein-coding genes, about 550 are regulated in a cell-cycle-dependent manner, in large part by three regulatory proteins: CtrA, GcrA and DnaA, which together control the expression of 185 cell-cycle regulated genes. CtrA upregulates the expression of many genes involved in cell division: DNA methylation, flagella, stalk, and septal Z-ring biogenesis. In addition, CtrA binds to five DNA sites that overlap with the binding sites of the replication initiation protein, DnaA, and thereby precludes a new round of DNA replication. Furthermore, CtrA inhibits the expression of GcrA, which functions as an activator of components of the replisome and the segregation machinery.

Based on experimental evidence, the 'CtrA - bistable' switch mechanism [1] is proposed for cell cycle control in this bacterium ^[1]. And a mathematical model [2] was constructed to interpret the detailed temporal dynamics of regulatory gene expression during the cell cycle and differentiation process of wild-type cells as well as several mutant strains. ^[2] This model presents a unified view of temporal and spatial regulation of protein activities during the asymmetric cell division cycle of Caulobacter. It helps to interpret phenotypes of known mutants and predict novel ones.

Caulobacter aging

Caulobacter was the first asymmetric bacterium shown to age. Reproductive senescence was measured as the decline in the number of progeny produced over time. ^{[3] [4]} A similar phenomenon has since been described in the bacterium Escherichia coli, which gives rise to morphologically similar daughter cells.^[5]

References

1. ^ Brazhnik, Paul; Tyson, John (2006). "Cell cycle control in bacteria and yeast: a case of convergent evolution?". Cell Cycle 5 (5): 522-9.
2. ^ Li, Shenghua; Brazhnik, Paul, Sobral, Bruno and Tyson, John (2008). "A Quantitative Study of the Division Cycle of Caulobacter crescentus Stalked Cells.". PLoS Computational Biology.
3. ^ Ackermann, Martin; Stephen C. Stearns, Urs Jenal (2003). "Senescence in a bacterium with asymmetric division". Science 300 (5627): 1920. doi:10.1126/science.1083532.
4. ^ Ackermann, Martin; Alexandra Schauerte, Stephen C. Stearns, Urs Jenal (2007). "Experimental evolution of aging in a bacterium". BMC Evolutionary Biology 7 (126).
5. ^ Stewart, Eric J.; Richard Madden, Gregory Paul, Francois Taddei (2005). "Aging and death in an organism that reproduces by morphologically symmetric division.". PLoS Biology 3 (2): e45. doi:10.1371/journal.pbio.0030045.