Structural effects of mutations in Salmonella typhimurium flagellar switch complex.

Mutations in Salmonella typhimurium fliG, fliM and fliN give rise either to non-flagellate, non-motile or non-chemotactic mutant bacteria. The FliG, FliM and FliN proteins form part of recently characterized extended flagellar basal structures, and have been postulated to form a mutually interacting structural complex. We have examined basal body preparations from non-motile or non-chemotactic fliG, fliM and fliN mutant strains by electron microscopy and immunoblot gel analysis. Most flagellar preparations isolated from the non-motile mutants lacked FliM, but contained FliG. The basal bodies lacked the belled morphology characteristic of the wild-type structures, but had protrusions which could be labelled with anti-FliG. Non-motile mutant preparations severely depleted of FliG but containing FliM were also obtained. These preparations contained extended, belled flagellar structures that were labelled with anti-FliM. Thus, FliM is part of the shell of the extended structures responsible for the belled morphology, while FliG may be part of the inner substructure. The extended basal structures from a FliG temperature-sensitive mutant strain rapidly lost FliM, as well as FliG, upon a shift to a non-permissive temperature, implying interaction between the FliG- and FliM-containing substructures. In dramatic contrast to non-motile mutants, extended basal structures isolated from non-chemotactic mutants were indistinguishable from wild-type structures. This difference may reflect the energetics of the different protein-protein interactions operative during torque generation and the switching of rotation sense.