Temperature-dependent FlgM/FliA complex formation regulates Campylobacter jejuni flagella length

Regulation of the biosynthesis of the flagellar filament in bacteria containing multiple flagellin genes is not well understood. The major food-borne pathogen Campylobacter jejuni possesses on both poles a flagellum that consists of two different flagellin subunits, FlaA and FlaB. Here we identify the protein Cj1464 as a regulator of C. jejuni flagellin biosynthesis. The protein shares characteristics of the FlgM family of anti-sigma factor proteins: it represses transcription of sigma(28)-dependent genes, forms a complex with sigma factor FliA, and is secreted through the flagellar filament. However, unlike other FlgM proteins, the interaction of C. jejuni FlgM with FliA is regulated by temperature and the protein does not inhibit FliA activity during the formation of the hook-basal body complex (HBB). Instead, C. jejuni FlgM limits the length of the flagellar filament by suppressing the synthesis of both the sigma(28)- and the sigma(54)-dependent flagellins. The main function of the C. jejuni FlgM therefore is not to silence sigma(28)-dependent genes until the HBB is completed, but to prevent unlimited elongation of the flagellum, which otherwise leads to reduced bacterial motility.