Bacterial flagellin-specific chaperone FliS interacts with anti-sigma factor FlgM

Flagella are extracellular organelles that propel bacteria. Each flagellum consists of a basal body, a hook, and a filament. The major protein of the filament is flagellin. To prevent premature polymerization of newly synthesized flagellin molecules, FliS, the flagellin-specific chaperone, binds flagellin and facilitates its export. Induction of flagellin gene expression coincides with secretion of FlgM. The role of FlgM is to inhibit FliA (Ơ28), a flagellar-specific RNA polymerase responsible for flagellin transcription. To prevent premature polymerization of newly synthesized flagellin molecules, FliS, the flagellin-specific chaperone, binds flagellin and facilitates its export. In this study, the interaction between FlgM and FliS from Salmonella typhimurium was characterized using gel-shift, intrinsic tryptophan fluorescence, circular dichroism, limited proteolysis and cross-linking. We have demonstrated that: i) FliS and FlgM interact specifically, forming a 1:1 complex; ii) The FliS binding site on FlgM is proximal to or even overlaps with the binding site for FliA; and iii) FliA competes with FliS for FlgM binding. We suggest that FliS binding by FlgM helps to stabilize and preserve FlgM in the cell until FliA is expressed.