Cloning and characterization of putative zinc protease genes of Ehrlichia canis

A putative zinc protease gene operon from Ehrlichia canis was cloned and sequenced. A genomic library was constructed in a pHG165 plasmid vector using Sau3A partially digested E. canis chromosomal DNA. Sequence analysis of the insert DNA from this clone indicated two open reading frames with a size of 1314 and 1350 bp that encodes for ProA and ProB, respectively. Based on BLAST analyses, ProA and ProB share 20-30% identities with members of the eukaryotic mitochondrial processing peptidase (MPP) subfamily, which are heterodimers containing alpha and beta subunits. The subunits share a 20% of identity, but only MPP-beta contains a conserved zincbinding motif, His-Xaa-Xaa-Glu-His (HXXEH). proA and proB are also detectable in E. canis and Ehrlichia chaffeensis, but not the Anaplasma phagocytophila. 5'-RACE revealed that the 5' end of the proA mRNA is heterogeneous, containing additional adenine residues that may be directed by pseudo-templated transcription. Although ProA was identified in both E. canis and E. chaffeensis, ProB was detected only in E. canis. ProA and ProB were both detectable in E. canis-infected DH82 cells. Sera from dogs, which were either naturally or experimentally infected with E. canis, recognized both the recombinant protein antigens.