Concerted evolution of duplicate fla genes in Campylobacter

Campylobacters have two similar copies (flaA and flaB) of their flagellin g ene. It has been hypothesized that the two copies can serve for antigenic p hase variation. Analysis of polymorphisms within aligned multiple DNA seque nces of the Campylobacter flagellin genes revealed high pairwise homoplasy indexes between flaB/flaB pairs that were not observed between any flaA/fla A pairings or flaA/flaB pairings. Thus it seems there are constraints on th e sequence of flaB that distinguish it from flaA. Nevertheless. segments of the two genes that are highly variable between strains are conserved betwe en the flaA and flaB copies of the genes within a strain. The patterns of s ynonymous and non-synonymous differences suggest that one segment of the fl agellin sequence is under selective pressure at the amino acid sequence lev el. Another segment of the protein is maintained within a strain by convers ion or recombination. Comparisons of strict consensus amino acid sequences did not reveal any motifs that are uniquely FlaA or FlaB, but there are dif ferences between FlaA and FlaB in those amino acids available for post-tran slational modification. The observed pattern of concerted evolution of port ions of a structural gene is an unusual finding in bacteria and should be s earched for with other duplicated genes. Concerted evolution was unexpected for genes involved in phase variation since it minimizes the antigenic rep ertoire that can be expressed by a single clone in the face of the host imm une response.