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 .

Signal transduction pathways mediated by sensor histidine kinases and cognate response regulators control a variety of physiological processes in response to environmental conditions. Here we show that in Caulobacter crescentus these systems also play essential roles in the regulation of polar morphogenesis and cell division. Previous studies have implicated histidine kinase genes pleC and divJ in the regulation of these developmental events. We now report that divK encodes an essential, cell cycle‐regulated homolog of the CheY/Spo0F subfamily and present evidence that this protein is a cognate response regulator of the histidine kinase PleC. The purified kinase domain of PleC, like that of DivJ, can serve as an efficient phosphodonor to DivK and as a phospho‐DivK phosphatase. Based on these and earlier genetic results we propose that PleC and DivK are members of a signal transduction pathway that couples motility and stalk formation to completion of a late cell division cycle event. Gene disruption experiments and the filamentous phenotype of the conditional divK341 mutant reveal that DivK also functions in an essential signal transduction pathway required for cell division, apparently in response to another histidine kinase. We suggest that phosphotransfer mediated by these two‐component signal transduction systems may represent a general mechanism regulating cell differentiation and cell division in response to successive cell cycle checkpoints.

Cell differentiation and division in Caulobacter crescentus are regulated by a signal transduction pathway mediated by the histidine kinase DivJ and the essential response regulator DivK. Here we report genetic and biochemical evidence that the DivJ and DivK proteins function to control the activity of CtrA, a response regulator required for multiple cell cycle events, including f lagellum biosynthesis, DNA replication, and cell division. Temperature-sensitive sokA (suppressor of divK) alleles were isolated as extragenic suppressors of a cold-sensitive divK mutation and mapped to the C terminus of the CtrA protein. The sokA alleles also suppress the lethal phenotype of a divK gene disruption and the cold-sensitive cell division phenotype of divJ mutants. The relationship between these signal transduction components and their target was further defined by demonstrating that the purified DivJ kinase phosphorylates CtrA, as well as DivK. Our studies also showed that phospho-CtrA activates transcription in vitro from the class II f lagellar genes and that their promoters are recognized by the principal C. crescentus sigma factor 73 . We propose that an essential signal transduction pathway mediated by DivJ, DivK, and CtrA coordinates cell cycle and developmental events in C. crescentus by regulating the level of CtrA phosphorylation and transcription from 73 -dependent class II gene promoters. Our results suggest that an unidentified phosphotransfer protein or kinase (X) is responsible for phosphoryl group transfer to CtrA in the proposed DivJ f DivK f X f CtrA phosphorelay pathway.Bacterial two-component signal transduction systems control a wide array of physiological processes in response to a variety of environmental conditions. These systems typically contain a sensor kinase, which is autophosphorylated on a conserved histidine residue, and a cognate response regulator containing a conserved aspartate residue to which the phosphoryl group is transferred (1, 2). This same protein family functions in multistep phosphorelay pathways (3). Recent results have provided evidence that sensor kinases and response regulators also play essential roles in the coordination of cell cycle and developmental events in the aquatic bacterium Caulobacter crescentus (4). The histidine kinase DivJ (5) and response regulator DivK (6) have been implicated in a signal transduction pathway required for cell division initiation. Another response regulator, CtrA, has been identified as a transcription factor responsible for the expression of multiple cell cycleregulated genes (7). Our results now indicate that DivJ and DivK proteins function as part of a multicomponent signal transduction pathway to control the transcriptional activity of CtrA during the cell cycle.Members of two-component signal transduction pathways regulating cell cycle events in C. crescentus were originally identified in a pseudoreversion analysis of pleC, a pleiotropic developmental gene required for motility and polar morphogenesis (8). Several of the pleC...