The pili of the stalked bacterium Caulobacter crescentus are assembled at a specific time in the life cycle at one pole of the cell and are composed of the monomer protein, pilin. A previous study demonstrated that the onset of pilin synthesis occurs well before pili appear on the surface, suggesting that pilin accumulates within the cell. In the present study, an electron microscope immunocytochemistry assay was used to determine the subcellular location of this unassembled pilin and its fate during pilus assembly and cell division. Populations of synchronously growing cells were embedded in epoxy resin at selected times during the cell cycle. Ultrathin sections were treated with pilin-specific antibody, followed by protein A coupled to colloidal gold. It was determined that the cellular location for unassembled pilin was the cell cytoplasm. All cell membranes and regions of nuclear material were poorly labeled. Quantitation demonstrated that label density increased during the period of pilin synthesis and declined during the period of pilus assembly and maintenance. The pilin pool was not unequally segregated at division; e.g., to the daughter cell that is elaborating pili. Mutants which have simultaneously lost the ability to produce flagella, pili, and other polar organelles, possibly due to alterations in the specialized region of polar organelle assembly, were also examined by the immunocytochemistry technique. There was no significant difference in the pilin pool size relative to the wild type, indicating that pilin synthesis continues in the absence of a functioning assembly site. This pattern of synthesis and assembly for the pilus is significantly different from that of the polar flagellum which is produced at the same time and location on the cell surface. These findings are discussed in relation to the hypothesized organization center at the cell pole which may have a major role in directing the assembly of all the polar structures.

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