Species-wide variation in the Escherichia coli flagellin (H-antigen) gene

Escherichia coli is a clonal species. The best-understood components of its clonal variation are the flagellar (H) and polysaccharide (O) antigens, both well documented since the mid-1930s because of their use in serotyping. Flagellin is the protein subunit of the flagellum that carries H-antigen specificity. We show that 43 of the 54 H-antigen specificities of E. coli map to the flagellin gene at fliC and sequenced all 43 forms and confirmed specificity of each by cloning and expression. This is, to our knowledge, the first time that all known forms of such a highly polymorphic gene have been fully sequenced and characterized for any species. The established distinction between a highly variable central region and more conserved flanking regions is upheld. The sequences fall into two groups, one of which may be derived from the fliC gene of the E. coli/Salmonella enterica common ancestor, the other perhaps obtained by lateral transfer since species divergence. Comparison of sequences revealed that both horizontal DNA transfer and fixation of mutations under diversifying selection pressure contributed to polymorphism in this locus.