CAULOBACTER FLAGELLAR FUNCTION, BUT NOT ASSEMBLY, REQUIRES FLIL, A NON-POLARLY LOCALIZED MEMBRANE-PROTEIN PRESENT IN ALL CELL-TYPES

Caulobacter crescentus has a single polar flagellum, which is assemble d in the predivisional cell. Known flagellar genes encode structural a nd regulatory components that are required for flagellar assembly and function. These genes are organized in several classes which form a tr anscriptional regulatory hierarchy. A member of the Class II genes, th e fliLM operon, encodes homologs of the Escherchia coli flagellar swit ch protein: FliM; and a protein with a hitherto unknown function, FliL . We report here that flagellar rotation requires the FliL protein. In -frame deletions in the chromosomal copy of the fliL gene result in ce lls that form a flagellum but are non-motile. The FliL protein was fou nd to be associated with the inner membrane and to be present in all c ell types. This is the first report of a Caulobacter crescentus protei n that is essential for motility but is not spatially restricted to th e region of the flagellar basal body. Although FliL is required for fl agellar function, it is not part of the transcriptional hierarchy, sup porting the hypothesis that, as is the case for the enterics, the regu latory hierarchy resp ends to assembly cues rather than directly to th e expression of flagellar proteins.