No abstract

The complete genome sequence of Caulobacter crescentus was determined to be 4,016,942 base pairs in a single circular chromosome encoding 3,767 genes. This organism, which grows in a dilute aquatic environment, coordinates the cell division cycle and multiple cell differentiation events. With the annotated genome sequence, a full description of the genetic network that controls bacterial differentiation, cell growth, and cell cycle progression is within reach. Two-component signal transduction proteins are known to play a significant role in cell cycle progression. Genome analysis revealed that the C. crescentus genome encodes a significantly higher number of these signaling proteins (105) than any bacterial genome sequenced thus far. Another regulatory mechanism involved in cell cycle progression is DNA methylation. The occurrence of the recognition sequence for an essential DNA methylating enzyme that is required for cell cycle regulation is severely limited and shows a bias to intergenic regions. The genome contains multiple clusters of genes encoding proteins essential for survival in a nutrient poor habitat. Included are those involved in chemotaxis, outer membrane channel function, degradation of aromatic ring compounds, and the breakdown of plant-derived carbon sources, in addition to many extracytoplasmic function sigma factors, providing the organism with the ability to respond to a wide range of environmental fluctuations. C. crescentus is, to our knowledge, the first free-living α-class proteobacterium to be sequenced and will serve as a foundation for exploring the biology of this group of bacteria, which includes the obligate endosymbiont and human pathogen Rickettsia prowazekii , the plant pathogen Agrobacterium tumefaciens , and the bovine and human pathogen Brucella abortus .

Mutations having pleiotropic effects on polar organelle development (pod) in Caulobacter crescentus have been identified and shown to occur in at least 13 genes scattered throughout the genome. Mutations at each locus affect a unique combination of polar traits, suggesting that complex interactions occur among these genes. The DNA sequence of one of these genes, pkeC, indites that it is homologous to members of the family of histidine protein kinase genes. Members of this family include the sensor components of the bacterial twocomponent regulatory systems. Furthermore, in vitro experiments demonstrated that the PleC protein was capable of autophosphorylation. These results suggest that the PieC protein (and perhaps the proteins encoded by the other pod genes as well) regulates the expes of genes involved in polar organelle development through the phosphorylation of key regulatory proteins. The use of a phosphorelay system cued to internal changes in the cell would provide a mechanis for coordinating major changes in gene expression with the completion of specific cell cycle events.Differentiation in Caulobacter crescentus is observed as an ordered series of events at the poles of the cell. Each cell division results in two cell types, a stalked cell like the parent and a motile swarmer cell. The two new poles formed at cell division lack polar organelles, while the older poles contain polar organelles produced during previous rounds of differentiation. After cell division, DNA replication is initiated immediately in the stalked progeny cell, followed by the assembly of a set of polar organelles including a flagellum, pili, and phage receptors at the newly created pole. The resulting predivisional cell has a different set of organelles at each pole, the original stalk at one end and the flagellum, pili, and phage receptors at the other.After a period of motility, the swarmer cell releases the flagellum into the culture medium, and the remainder of the polar organelles are replaced by a stalk. An exception is the holdfast material, which is found at the base of the flagellum.