The organization of the Caulobacter crescentus flagellar filament

The structural organization of the flagellar filament of Caulobacter crescentus, as revealed by immunoelectron microscopy, shows five antigenically distinct regions within the hook-filament complex. The first region is the hook. The second region is adjacent to the hook and is approximately 10 nm in length. On the basis of its location in the hook-filament complex, this region may contain hook-associated proteins. Next to this is the third region, which is approximately 60 nm in length. Antibody decoration experiments using mutant strains with deletions of the structural gene for the 29 × 10³ M_r flagellin (flgJ) showed that the presence of this region is correlated with the expression of the 29 × 10³ M_r flagellin gene. The next region (region IV), of length ~1 to 2 μm, appears to contain the 27·5 × 10³ M_r flagellin, but at its distal end includes, in gradually increasing amounts, the 25 × 10³ M_r, flagellin. The rest of the filament (region V) is made up predominantly, if not completely, of the 25 × 10³ M_r flagellin. Except for the hook, there are no morphological features that would otherwise distinguish these regions. A functional flagellum, having the wild-type length and morphology, is assembled by mutant strains deficient in the 29 × 10³ M_r flagellin and 27·5 × 10³ M_r flagellin.