Groupings of highly similar major surface protein (p44)-encoding paralogues:: a potential index of genetic diversity amongst isolates of Anaplasma phagocytophilum

Anaplasma phagocytophilum is a tick-borne bacterium that is zoonotic in the USA and southern Europe, but although the bacterium is endemic in the UK, no cases of clinical human disease have yet been detected in that country. Potential genomic differences amongst UK and USA isolates were investigated by comparing partial 16S rRNA gene and p44 paralogue sequences amplified by PCR from 10 UK ruminant or tick isolates, with published sequences from USA isolates. No significant clustering among the isolates was resolved by phylogenetic analysis of alignments containing 16S rRNA gene sequences. The structure of predicted proteins encoded by p44 paralogues, amplified from 81 clones obtained from the UK isolates, was similar to that described previously for paralogues from USA isolates. Paralogue sequences did not obviously cluster by country, host species or isolate, but most paralogues were 30-70% similar, making meaningful alignments difficult. Some p44 paralogues from different isolates formed clusters of sequences that were more than 90% similar to one another ('similarity groups'). The paralogues in each cluster were particularly similar in gene regions most likely to code for ligands. In the sample studied, 95% of the similarity groups comprised paralogues from either USA or UK isolates only and occurred with greater frequency amongst paralogues from USA rather than UK isolates. These findings raise the hypothesis that sequences of paralogues in similarity groups may provide an index of adaptation of different 'strains' of A. phagocytophilum to specific reservoir hosts in different geographical locations, and any associations with infectivity for different species including humans.