Defining the Pathway to Bacterial Polarity

Abstract

Caulobacter crescentus is a flagellated bacterium that undergoes a transition from a motile swarmer cell to a stalked cell during cell division, which produces a new swarmer cell and the stalked cell. After division, the stalked cell can initiate DNA replication immediately, but the new swarmer cell must undergo the transition to a stalked cell in order to divide. During the swarmer-to-stalked cell transition (which corresponds to G 1 -to-S phase transition) and just before cell separation (predivisional cells), the response regulator and transcriptional regulator CtrA, which represses chromosome replication among other functions, must be localized to the stalked cell pole and degraded. Ryan et al. investigated the mechanism whereby CtrA polarity is established. Expression of various mutants fused to the green fluorescent protein variant (YFP) followed by microscopic analysis showed that neither phosphorylation nor proteolysis was required for proper polar localization of CtrA. The signal for polarization was located in the receiver domain. The authors also analyzed CtrA localization in cells carrying mutations in kinases (DivJ and PleC) responsible for regulating the phosphorylation status of response regulator DivK, which is necessary for CtrA proteolysis. Although DivK is localized to the stalked pole, a site of CtrA accumulation, it also localizes to the swarmer cell pole, a site that does not accumulate CtrA. Analysis of the divK , divJ , and pleC mutant strains indicated that polar localization of DivK was not required for stalked pole localization of CtrA--some cells maintained polar accumulation of CtrA, but aberrations in localization or timing of the polarity were also observed--but that these three components participate in a pathway that allows the phosphorylation state of DivK to dictate the activity of DivK and the asymmetric proteolysis of CtrA. Holtzendorff et al. have identified a global cell cycle regulator, GcrA, in Caulobacter crescentus , which forms an oscillatory circuit with the CtrA. GcrA depletion is lethal. Phosphorylated CtrA repressed the gcrA gene, and the abundance of GcrA was in inverse proportion to that of CtrA, such that in stalked cells, in which CtrA is proteolyzed, GcrA accumulated. GcrA in turn stimulated ctrA expression and was a direct transcriptional regulator that bound to the ctrA promoter. Microarray experiments indicated that GcrA regulated 125 genes, including those encoding PleC, DivK, and CtrA. Thus, GcrA and CtrA directly regulate each other so that their concentrations oscillate out of phase temporally and spatially during the cell cycle to drive cell cycle progression and expression of polar differentiation factors. K. R. Ryan, S. Huntwork, L. Shapiro, Recruitment of a cytoplasmic response regulator to the cell pole is linked to its cell cycle-regulated proteolysis. Proc. Natl. Acad. Sci. U.S.A. 101 , 7415-7420 (2004). [Abstract] [Full Text] J. Holtzendorff, D. Hung, P. Brende, A. Reisenauer, P. H. Viollier, H. H. McAdams, L. Shapiro, Oscillating global regulators control the genetic circuit driving a bacterial cell cycle. Science 304 , 983-987 (2004). [Abstract] [Full Text]