Sequence diversity of flagellin (fliC) alleles in pathogenic Escherichia coli

To study the molecular evolution of flagellin, the protein subunit specifyi ng flagellar (H) antigens, the fliC genes from 15 pathogenic strains of Esc herichia coli were amplified by PCR and sequenced. Comparison of fliC seque nces of H6 and H7 strains revealed that alleles have a mosaic structure ind icating the occurrence of past horizontal transfer of DNA segments between strains. The close similarity of H7 sequences also indicates the exchange o f an entire fliC H7 allele between distant clonal lineages. In addition, th e ratio of silent substitutions to amino acid replacements suggests that a short segment in the central region of fliC has been under positive selecti on in the divergence of H6 and H7 alleles. Phylogenetic analysis demonstrat es that the fliC sequences of O157:H7 and O55:H7 serotypes are nearly ident ical and highly divergent from those of E. coli strains expressing H6 and H 2 flagellar antigens. A nonmotile clone of sorbitol-fermenting O157 has rap idly accumulated multiple mutations in fliC, presumably as a result of the silencing of flagellin expression.