The interaction of protein structure, selection, and recombination on the evolution of the type-1 fimbrial major subunit (fimA) from Escherichia coli

Fimbrial adhesins allow bacteria to interact with and attach to their envir onment. The bacteria possibly benefit from these interactions, but all exte rnal structures including adhesins also allow bacteria to be identified by other organisms. Thus adhesion molecules might. be under multiple forms of selection including selection to constrain functional interactions or evolv e novel epitopes to avoid recognition. We address these issues by studying genetic diversity in the Escherichia coli type-1 fimbrial major subunit, fi mA. Overall, sequence diversity in fimA is high (pi = 0.07) relative to tha t in other E. coli genes. High diversity is a function of positive diversif ying selection, as detected by d(N)/d(S) ratios higher than 1.0, and amino acid residuces subject to diversifying selection are nonrandomly clustered on the exterior surface of the peptide, In addition, McDonald and Kreitman tests suggest that there has been historical but not current directional se lection at fimA between E. coli and Salmonella. Finally, some regions of th e fimA peptide appear to be under strong structural constraint within E. co li, particularly the interior regions of the molecule that is involved in s ubunit to subunit interaction. Recombination also plays a major role contri buting to E, coli fimA allelic variation and estimates of recombination (2N (e)c) and mutation (2N(e)mu) are about the same. Recombination may act to s eparate the diverse evolutionary forces in different regions of the fimA pe ptide.