Polymorphism and transcription at the p44-1/p44-18 genomic locus in Anaplasma phagocytophilum strains from diverse geographic regions.

A polymorphic multigene family (p44) of Anaplasma phagocytophilum encodes the immunodominant 44-kDa major outer membrane proteins. With p44-specific PCR and gene-specific probes, p44-1 was found in all human isolates from New York State but not in isolates from Minnesota, whereas p44-18 and two other p44 species were found in isolates from both regions. We therefore sequenced the genomic locus corresponding to the p44-1/p44-18 tandem locus of A. phagocytophilum HZ in 14 other geographically divergent strains from various hosts. The locus was found in all 14 strains, and p44-18 was conserved among all 13 United States isolates studied. In all nine northeastern strains, p44-1 was conserved. However, in three of the Minnesota strains and in one California strain, p44-1 was replaced at this genomic locus by the novel gene p44-61 (p44-61/18), whose hypervariable region (hv) was a chimera of p44-20hv and p44-23hv. The conserved base sequence within the hv region linked the two segments. In contrast, in the Old Sourhope strain isolated from sheep in the United Kingdom, only a single and distinct p44, p44-OS, was found in this locus. This suggests different rates of evolution of p44-1 and p44-18 at this locus and conservation of the locus within strains isolated from the same geographic region. Locus-specific reverse transcription-PCR revealed expression of p44-1 by New York and p44-61 by Minnesota strains at this locus. These p44 loci provide insight into the molecular evolution and functional divergence of p44 paralogs and may serve as markers for typing strains from different geographic regions.