Abstract
Rickettsia peacockii, also known as the East Side Agent, is a non-pathogenic obligate intracellular bacterium found as an endosymbiont in Dermacentor andersoni ticks in the western USA and Canada. Its presence in ticks is correlated with reduced prevalence of Rickettsia rickettsii, the agent of Rocky Mountain Spotted Fever. It has been proposed that a virulent SFG rickettsia underwent changes to become the East Side Agent. We determined the genome sequence of R. peacockii and provide a comparison to a closely related virulent R. rickettsii. The presence of 42 chromosomal copies of the ISRpe1 transposon in the genome of R. peacockii is associated with a lack of synteny with the genome of R. rickettsii and numerous deletions via recombination between transposon copies. The plasmid contains a number of genes from distantly related organisms, such as part of the glycosylation island of Pseudomonas aeruginosa. Genes deleted or mutated in R. peacockii which may relate to loss of virulence include those coding for an ankyrin repeat containing protein, DsbA, RickA, protease II, OmpA, ScaI, and a putative phosphoethanolamine transferase. The gene coding for the ankyrin repeat containing protein is especially implicated as it is mutated in R. rickettsii strain Iowa, which has attenuated virulence. Presence of numerous copies of the ISRpe1 transposon, likely acquired by lateral transfer from a Cardinium species, are associated with extensive genomic reorganization and deletions. The deletion and mutation of genes possibly involved in loss of virulence have been identified by this genomic comparison. It also illustrates that the introduction of a transposon into the genome can have varied effects; either correlating with an increase in pathogenicity as in Francisella tularensis or a loss of pathogenicity as in R. peacockii and the recombination enabled by multiple transposon copies can cause significant deletions in some genomes while not in others.
Highlights
Rickettsia peacockii is an obligate intracellular bacterium identified in Rocky Mountain wood ticks (Dermacentor andersoni) from Montana, USA [1]
It is of interest to rickettsiologists due to its co-localization on the eastern side of the Bitterroot Valley with a much reduced prevalence of D. andersoni infected with Rickettsia rickettsii, while spotted fever ravaged the west side of the valley [2] [3]
Began the study of a phenomenon considered as evidence for interference, where the presence of R. peacockii in D. andersoni ticks may prevent the transovarial transmission of R. rickettsii and limit its spread in the tick population
Summary
Rickettsia peacockii is an obligate intracellular bacterium identified in Rocky Mountain wood ticks (Dermacentor andersoni) from Montana, USA [1]. Began the study of a phenomenon considered as evidence for interference, where the presence of R. peacockii in D. andersoni ticks may prevent the transovarial transmission of R. rickettsii and limit its spread in the tick population. It is not clear whether this interference is an active process or a case in which ticks carrying R. peacockii have a reproductive advantage because they do not suffer the reduced fecundity associated with R. rickettsii infection [4]. While R. peacockii is closely related to R. rickettsii, it is not a pathogen of mammals and not deleterious to ticks
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