Turning off flagellum rotation requires the pleiotropic gene pleD: pleA, pleC, and pleD define two morphogenic pathways in Caulobacter crescentus.

We have identified mutations in three pleiotropic genes, pleA, pleC, and pleD, that are required for differentiation in Caulobacter crescentus. pleA and pleC mutants were isolated in an extensive screen for strains defective in both motility and adsorption of polar bacteriophage phi CbK; using temperature-sensitive alleles, we determined the time at which the two genes act. pleA was required for a short period at 0.7 of the swarmer cell cycle for flagellum biosynthesis, whereas pleC was required during an overlapping period from 0.6 to 0.95 of the cell cycle to activate flagellum rotation as well as to enable loss of the flagellum and stalk formation by swarmer cells after division. The third pleiotropic gene, pleD, is described here for the first time. A pleD mutation was identified as a bypass suppressor of a temperature-sensitive pleC allele. Strains containing this mutation were highly motile, did not shed the flagellum or form stalks, and retained motility throughout the cell cycle. Since pleD was required to turn off motility and was a bypass suppressor of pleC, we conclude that it acts after the pleA and pleC gene functions in the cell cycle. No mutants defective in both flagellum biosynthesis and stalk formation were identified. Consequently, we propose that the steps required for formation of swarmer cells and subsequent development into stalked cells are organized into at least two developmental pathways: a pleA-dependent sequence of events, responsible for flagellum biosynthesis in predivisional cells, and a pleC-pleD-dependent sequence, responsible for flagellum activation in predivisional cells and loss of motility and stalk formation in progeny swarmer cells.