FLAGELLAR FILAMENT ELONGATION CAN BE IMPAIRED BY MUTATIONS IN THE HOOK PROTEIN FLGE OF SALMONELLA-TYPHIMURIUM - A POSSIBLE ROLE OF THE HOOKAS A PASSAGE FOR THE ANTI-SIGMA FACTOR FLGM

Among motile revertants isolated from flagellar hook-deficient (flgE) mutants of Salmonella typhimurium, one produced only short flagellar f ilaments in L broth, despite the fact that flagellin itself has the ab ility to polymerize into long filaments in vitro, This pseudo-revertan t has an intragenic suppressor, resulting in a two-amino-acid substitu tion (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 filame nts in the absence of NaCl, short filaments in 1% NaCl and full-length filaments in 2% NaCl, Electron microscopy of osmotically shocked cell s showed that the number of hook-basal bodies on cells was constant un der various NaCl conditions. Furthermore, we found that the mutant hoo k was straight rather than curved, We monitored the cellular flagellin level of this pseudorevertant under various NaCl concentrations by im munoblotting, It was revealed that little flagellin was present under NaCl-free conditions in contrast with the ordinary amounts of flagelli n present in 2% NaCl. As the expression of flagellin is regulated by c ompetitive interaction of a sigma factor, FliA, and a corresponding an ti-sigma factor, FlgM, we also observed the effect of NaCl on the secr etion of FlgM, FlgM was secreted into the media in more than 1% NaCl b ut accumulated inside the cells in the absence of NaCl, indicating tha t the failure of secretion of FlgM in the absence of salt was the caus e of the impaired elongation of filaments.