Identification of a novel response regulator required for the swarmer-to-stalked-cell transition in Caulobacter crescentus

The onset of motility late in the Caulobacter crescentus cell cycle depends on a signal transduction pathway mediated by the histidine kinase PleC and response regulator DivK. We now show that pleD, whose function is required for the subsequent loss of motility and stalk formation by the motile swarmer cell, encodes a 454-residue protein with tandem N-terminal response regulator domains D1 and D2 and a novel C-terminal GGDEF domain. The identification of pleD301, a semidominant suppressor of the pleC Mot phenotype, as a mutation predicted to result in a D-533G change in the D1 domain supports a role for phosphorylation in the PleD regulator. Disruptions constructed in the pleD open reading frame demonstrated that the gene is not essential and that the pleC phenotype can also be suppressed by a recessive, loss-of-function mutation. These results suggest that PleD is part of a signal transduction pathway controlling stalked-cell differentiation early in the C. crescentus cell cycle. Caulobacter crescentus is an asymmetrically dividing bacte-rium, which produces a nonmotile stalked mother cell and a motile, flagellated swarmer cell at the end of each cell cycle. During cell division, the stalked cell undergoes a series of discrete developmental events at one pole, including flagellu