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Insights into genome evolution: The sequence of Rickettsia conorii
By Birgit Reinert

French researchers have sequenced the genome of Rickettsia conorii, a bacterium that causes Mediterranean spotted fever in humans when it is transmitted by brown dog ticks. The bacterium is closely related to R. prowazekii, which causes typhus and whose genome was sequenced in 1998. Both bacteria are parasites with a relatively small genome, in part because they tend to inactivate their own genes when they can make use of genes from their host.

Male and female brown dog tick (Rhipicephalus sanquineus).

Didier Raoult, of the Université de la Mediterranée in Marseille, France, and colleagues sequenced the Malish 7 strain of R. conorii. Both Rickettsia genomes are much smaller than they were at the point of their divergence from a common ancestor millions of years ago, according to a comparative analysis by the researchers. This supports the theory that small genome size is useful for preserving the organisms' energy and improving its efficiency.

"We saw evidence of the very gradual nature of the genome reduction process by identifying all possible intermediates," the researchers write in Science. The presence of genes in different stages of transformation or decay is an indication that both parasites may have been adapting to their particular host environment rather than simply losing genes at random.

"Similar mechanisms probably occur in the evolution of all bacterial species but have remained undetected because of more active recombination and a faster evolutionary rate," the researchers write. Their analysis indicates that R. prowazekii, the smaller of the two bacteria, seems to be evolving more rapidly.

The R. conorii genome has 1.3 million base pairs and 1,374 genes, slightly more than its cousin. The overlap for parts of the genomes is striking: All but thirty genes from a set of 834 in R. prowazekii have counterparts in R. conorii.

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Ogata, H. et al. Mechanisms of evolution in Rickettsia conorii and R. prowazekii. Science 293, 2093-2098 (September 14, 2001).
Andersson, S.G. et al. The genome sequence of Rickettsia prowazekii and the origin of mitochondria. Nature 396, 133-140 (November 12, 1998).

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