Three-dimensional structure of the complex flagellar filament of Rhizobium lupini and its relation to the structure of the plain filament.

Electron micrographs of negatively stained preparations were used to obtain a three-dimensional reconstruction of the complex flagellar filament of Rhizobium lupini H13-3. The complex filament has an organization similar to that of the more common plain filament, but the subunits are perturbed in a pairwise fashion to generate a very distinctive set of three continuous ridges of density along the outer surface of the filament. In the three-dimensional map, the design of the complex filament is similar to that of the plain filament described in the accompanying paper. The structures consist of 11 segmented rods of density lying at a radius of 65 to 70 A. The exterior surfaces of both kinds of filaments consist of features that protrude from the segmented rods. The interiors of both consist of arms that extend inwards from the rods. In the case of the complex filament, but not of the plain filament, the inner arms interact to generate three tubular features, which, together with the three outer ridges, may account for the more brittle and, by implication, stiffer nature of the complex filament.