Diversifying selection governs sequence polymorphism in the major adhesin proteins fimA, papA, and sfaA of Escherichia coli

Fimbriae or pili are essential adherence factors usually found in pathogenic bacteria to aid colonization of host cells. Three major structural pilin genes, fimA, sfaA, and papA, from Escherichia coli natural isolates were examined and nucleotide sequence data revealed elevated levels of both synonymous and nonsynonymous site variation at these loci. Examination of synonymous site variation shows a fivefold increase in fimA sites, relative to the housekeeping gene mdh; and similarly the sfaA and papA genes have increased synonymous sites variation relative to fimA. Nonsynonymous site variation is also elevated at all three loci but, in particular, at the papA locus (kN = 0.44). The kN/kS ratio for the three genes are among the highest yet reported for E. coli genes. Regional variation in nucleotide polymorphism within each of the genes reveal hypervariable segments where nonsynonymous substitutions exceed synonymous substitutions. We propose that at the fimA, papA, and sfaA genes, diversifying selection has brought about the increase levels of polymorphism.