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A Caulobacter gene involved in polar morphogenesis

At specific times in the cell cycle, the bacterium Caulobacter crescentus assembles two major polar organelles, the flagellum and the stalk. Previous studies have shown that flbT mutants overproduce flagellins and are unable to form chemotaxis swarm rings. In this paper, we report alterations in both the stalk and the flagellar structure that result from a mutation in the flagellar gene flbT. Mutant strains produce some stalks that have a flagellum, produce some stalks that have an extra lobe protruding from their sides, have filaments lacking the 29-kilodalton flagellin, and produce several unusual cell types, including filamentous cells as well as predivisional cells with two stalks and predivisional cells with no stalk at all. We propose that flagellated stalks arise as a consequence of a failure to eject the flagellum at the correct time in the cell cycle and that the extra stalk lobe is due to a second site for the initiation of stalk biogenesis. Thus, a step in the pathway that establishes the characteristic asymmetry of the C. crescentus cell appears to be disrupted in flbT mutants. We have also identified a new structural feature at the flagellated pole and the tip of the stalk: the 10-nm polar particle. The polar particles appear as a cluster of approximately 1 to 10 stain-excluding rings, visible in electron micrographs of negatively stained wild-type cells. This structure is absent at the flagellar pole but not in the stalks of flbT mutant predivisional cells.