Formation of a single polar flagellum by two distinct flagellar gene sets in Sphingomonas sp. strain A1

Gram-negative Sphingomonas sp. strain A1, originally identified as a non-motile and aflagellate bacterium, possesses two sets of genes required for flagellar formation. In this study, we characterized the flagellar genes and flagellum formation in strain A1. Flagellar gene cluster set I contained 35 flagellar genes, including one flagellin gene (p6), where the gene assembly structure resembled that required for the formation of lateral flagella in gammaproteobacteria. The set II flagellar genes were arranged in eight shorter clusters with 46 flagellar genes, including two flagellin genes (p5 and p5') and flhF, which is required for polar flagella. Our molecular phylogenetic analysis of the bacterial flagellins also demonstrated that, in contrast to p5 and p5', p6 was categorized as a lateral flagellin group. The motile phenotype appeared in strain A1 cells when they were subcultured on semisolid media. The motile strain A1 cells produced a single flagellum at the cell pole. DNA microarray analyses using non-motile and motile strain A1 cells indicated that flagellar formation was accompanied by increased transcription of both flagellar gene sets. The two flagellins p5 and p6 were major components of the flagellar filaments isolated from motile strain A1 cells, indicating that the polar flagellum is formed by lateral and non-lateral flagellins.