New perspectives on rickettsial evolution from new genome sequences of Rickettsia, particularly R. canadensis, and Orientia tsutsugamushi

The complete genome sequences available for eight species of Rickettsia and information for other near relatives in the Rickettsiales including Orientia and species of Anaplasmataceae are a rich resource for comparative analyses of the evolution of these obligate intracellular bacteria. Differences in these organisms have permitted them to colonize varied intracellular compartments, arthropod vectors, and vertebrate reservoirs in both pathogenic and symbiotic relationships. We summarize some comparative aspects of the genomes of these organisms, paying particular attention to the recently completed sequence for R. canadensis McKiel strain and an estimated two-thirds of the genome sequence for a Thailand patient isolate of Orientia tsutsugamushi. The Rickettsia genomes exhibit a high degree of synteny punctuated by distinctive chromosome inversions and consistent phylogenetic relationships regardless of whether protein coding sequences or RNA genes, concatenated open reading frames or gene regions, or whole genomes are used to construct phylogenetic trees. The aggregate characteristics (number, length, composition, repeat identity) of tandem repeat sequences of Rickettsia, which often exhibit recent and rapid divergence between closely related strains and species of bacteria, are also very conserved in Rickettsia but differed significantly in Orientia. O. tsutsugamushi shared no significant synteny to species of Rickettsia or Anaplasmataceae, supporting its placement in a unique genus. Like Rickettsia felis, Orientia has many transposases and ankyrin and tetratricopeptide repeat domains. Orientia shares the important ATP/ADP translocase and proline-betaine transporter multigene families with Rickettsia, but has more gene families that may be involved in regulatory and transporter responses to environmental stimuli.