Flagellar filament elongation can be impaired by mutations in the hook protein FlgE of Salmonella typhimurium: a possible role of the hook as a passage for the anti-sigma factor FlgM

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Among motile revertants isolated from flagellar hook-deficient (flgE) mutants of Salmonella typhimurium, one produced only short flagellar filaments in L broth, despite the fact that flagellin itself has the ability to polymerize into long filaments in vitro. This pseudorevertant has an intragenic suppressor, resulting in a two-amino-acid substitution (Asp-Gln→Ala-Arg) in the C-terminal region of the hook protein, FlgE. The flagellation of the pseudorevertant was greatly affected by the concentration of NaCl in the culture media: we observed no filaments in the absence of NaCl, short filaments in 1% NaCl and full-length filaments in 2% NaCl. Electron microscopy of osmotically shocked cells showed that the number of hook-basal bodies on cells was constant under various NaCl conditions. Furthermore, we found that the mutant hook was straight rather than curved. We monitored the cellular flagellin level of this pseudorevertant under various NaCl concentrations by immunoblotting. It was revealed that little flagellin was present under NaCl-free conditions in contrast with the ordinary amounts of flagellin present in 2% NaCl. As the expression of flagellin is regulated by competitive interaction of a sigma factor, FliA, and a corresponding anti-sigma factor, FlgM, we also observed the effect of NaCl on the secretion of FlgM. FlgM was secreted into the media in more than 1% NaCl but accumulated inside the cells in the absence of NaCl, indicating that the failure of secretion of FlgM in the absence of salt was the cause of the impaired elongation of filaments.

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