A lytic transglycosylase homologue, PleA, is required for the assembly of pili and the flagellum at the Caulobacter crescentus cell pole

Two distinct protein complexes, the flagellum and the pilus biogenesis machinery, are asymmetrically assembled at one pole of the Caulobacter predivisional cell. Cell division yields dissimilar daughter cells: a stalked cell and a swarmer cell that assembles several pill at the flagellated cell pole. Strains bearing mutations in the pleA gene are pililess and non-flagellated. The PleA protein contains a region that is similar to a peptidoglycan-hydrolytic active site, and a point mutation at this site in PleA results in the loss of flagellum and pill biogenesis. PleA was found to be required for the insertion of the outer membrane pilus secretion channel at the cell pole and for the accumulation of the PilA pilin subunit. PleA is also required for the assembly of substructures of the flagellar basal body hook complex that are located in or traverse the peptidoglycan layer. These results argue that PleA facilitates the assembly of envelope-spanning structures at the cell pole. In support of this, PleA was found to be present only during a short interval in the cell cycle that coincides with the assembly of the flagellum and the pilus secretion apparatus.