Mechanisms of evolution in Rickettsia conorii and R. prowazekii

Rickettsia conorii is an obligate intracellular bacterium that causes Medit erranean spotted fever in humans. We determined the 1,268,755-nucleotide co mplete genome sequence of R. conorii, containing 1374 open reading frames. This genome exhibits 804 of the 834 genes of the previously determined R. p rowazekii genome plus 552 supplementary open reading frames and a 10-fold i ncrease in the number of repetitive elements. Despite these differences, th e two genomes exhibit a nearly perfect colinearity that allowed the clear i dentification of different stages of gene alterations with gene remnants an d 37 genes split in 105 fragments, of which 59 are transcribed. A 38-kiloba se sequence inversion was dated shortly after the divergence of the genus.