Lateral-typed flagellin responsible for formation of a polar flagellum but not of lateral flagella in Sphingomonas sp. strain A1.

Alginate-assimilating Sphingomonas sp. strain A1 is the gram-negative bacterium first identified to form a single polar flagellum containing lateral-typed flagellin (p6) in the filament. In addition to the p6 flagellin, two polar-typed flagellins (p5 and p5') are also included in the flagellum. Here we show the significant role of p6 as well as p5/p5' in flagellum formation and cell motility toward alginate. A p6 gene disruptant significantly reduced flagellum formation, and it showed no cell motility, whereas each mutant with a disruption in the p5 or p5' gene exhibited cell motility through the formation of a polar flagellum containing p6. The ratio of p6 to p5 decreased in proportion to cell growth, suggesting that strain A1 changes flagellin ratios in the filament depending on the external environment. Each of purified recombinant p5 and p6 proteins formed the filament by in vitro self-assembly, and an anti-p5 antibody reacted with the p5 filament but not with the p6 filament. Immunoelectron microscopy using an anti-p5 antibody indicated that strain A1 formed two types of the filament in a single polar flagellum: p6 alone in the entire filament and p5 elongation filament subsequent to the p6 proximal end. Immunoprecipitation with an anti-p5 antibody directly demonstrated that p5 and p6 coexist in a single filament. Strain A1 cells were also found to exhibit a chemotactic motility in response to alginate. This is the first report on function/location of the lateral-typed flagellin in a single polar flagellum and the bacterial chemotaxis toward alginate.