Protein Sequences and Cellular Factors Required for Polar Localization of a Histidine Kinase in Caulobacter crescentus

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Abstract
The Caulobacter crescentus sensor kinase DivJ is required for an early cell division step and localizes at the base of the newly formed stalk during the G1-to-S-phase transition when the protein is synthesized. To identify sequences within DivJ that are required for polar localization, we examined the ability of mutagenized DivJ sequences to direct localization of the green fluorescent protein. The effects of overlapping C-terminal deletions of DivJ established that the N-terminal 326 residues, which do not contain the kinase catalytic domain, are sufficient for polar localization of the fusion protein. Internal deletions mapped a shorter sequence between residues 251 and 312 of the cytoplasmic linker that are required for efficient localization of this sensor kinase. PleC kinase mutants, which are blocked in the swarmer-to-stalked-cell transition and form flagellated, nonmotile cells, also fail to localize DivJ. To dissect the cellular factors involved in establishing subcellular polarity, we have examined DivJ localization in a pleC mutant suppressed by the sokA301 allele of ctrA and in a pleD mutant, both of which display a supermotile, stalkless phenotype. The observation that these Mot+ strains localize DivJ to a single cell pole indicate that localization may be closely coupled to the gain of motility and that normal stalk formation is not required. We have also observed, however, that filamentous parC mutant cells, which are defective in DNA segregation and the completion of cell separation, are motile and still fail to localize DivJ to the new cell pole. These results suggest that formation of new sites for DivJ localization depends on events associated with the completion of cell separation as well as the gain of motility. Analysis of PleC and PleD mutants also provides insights into the function of the His-Asp proteins in cell cycle regulation. Thus, the ability of the sokA301 allele of ctrA to bypass the nonmotile phenotype of the pleC null mutation provides evidence that the PleC kinase controls cell motility by initiating a signal transduction pathway regulating activity of the global response regulator CtrA. Analysis of the pleD mutant cell cycle demonstrates that disruption of the swarmer-to-stalked-cell developmental sequence does not affect the asymmetric organization of the Caulobacter cell cycle.