The genome of the obligately intracellular bacterium Ehrlichia canis reveals themes of complex membrane structure and immune evasion strategies

Abstract

Ehrlichia canis, a small obligately intracellular, tick-transmitted, gram-negative, alpha-proteobacterium, is the primary etiologic agent of globally distributed canine monocytic ehrlichiosis. Complete genome sequencing revealed that the E. canis genome consists of a single circular chromosome of 1,315,030 bp predicted to encode 925 proteins, 40 stable RNA species, 17 putative pseudogenes, and a substantial proportion of noncoding sequence (27%). Interesting genome features include a large set of proteins with transmembrane helices and/or signal sequences and a unique serine-threonine bias associated with the potential for O glycosylation that was prominent in proteins associated with pathogen-host interactions. Furthermore, two paralogous protein families associated with immune evasion were identified, one of which contains poly(G-C) tracts, suggesting that they may play a role in phase variation and facilitation of persistent infections. Genes associated with pathogen-host interactions were identified, including a small group encoding proteins (n = 12) with tandem repeats and another group encoding proteins with eukaryote-like ankyrin domains (n = 7).

FIG. 1.

Circular representation of the genome of E. canis. From outside to inside, the first two circles represent COG assignment for predicted coding sequences on the plus and minus strands, respectively. Colors indicate the following: dark gray, hypothetical proteins; light gray, conserved hypothetical and unknown function; brown, general function prediction; red, replication and repair; green, energy metabolism; blue, carbon and carbohydrate metabolism; cyan, lipid metabolism; magenta, transcription; yellow, translation; orange, amino acid metabolism; pink, metabolism of cofactors and vitamins; light red, purine and pyrimidine metabolism; lavender, signal transduction; and sky blue, cellular processes. The third and fourth circles represent RNA genes. The fifth and sixth circles represent unique coding sequences. The two innermost circles represent the %G+C content and G+C skew values, respectively.

FIG. 2.

Synteny plots between E. canis (horizontal axis) and E. ruminantium (strain Gardel), A. marginale, and R. conorii. The numbers represent base pairs.

FIG. 3.

Venn diagram showing comparison of conserved and unique genes in Ehrlichia canis (yellow), Ehrlichia spp. (blue), Wolbachia spp. (pink), Rickettsia spp. (red), and Anaplasma spp. (green). The dark circle represents genes unique in E. canis.