Sequential regulation of developmental events during polar morphogenesis in Caulobacter crescentus: assembly of pili on swarmer cells requires cell separation.

Pili, along with the flagellum and DNA bacteriophage receptors, are structural markers for polar morphogenesis in Caulobacter crescentus. Pili act as primary receptors for a number of small, C. crescentus-specific DNA and RNA bacteriophages, and the timing of pilus-dependent adsorption of bacteriophage phiCb5 in synchronized cell populations has led to the general conclusion that pili are formed coordinately with the flagellum and other polar surface structures in the predivisional cell. The use of rotary platinum shadow casting and electron microscopy as a direct assay for formation of flagella and pili in synchronous cell cultures now shows, however, that when expressed as fractions of the swarmer cell cycle, flagella are assembled on the predivisional cells at approximately 0.8 and that pili are assembled on the new swarmer cells at approximately 0.1 of the next cell cycle. Adsorption of pilus-specific bacteriophage phiCb5 prevented the loss of pili from swarmer cells during development, which suggests that these structures are retracted at the time of stalk formation. Examination of temperature-sensitive cell division mutants showed that the assembly of pili depends on completion of cell separation. These results indicate that the stage-specific events required for polar morphogenesis in C. crescentus occur sequentially, rather than coordinately in the cell cycle, and that the timing of these events reflects the order of underlying cell cycle steps.