Molecular phylogeny and rearrangement of rRNA genes in Rickettsia species

It has previously been observed that Rickettsia prowazekii has an unusual a rrangement of the rRNA genes. In this species, the three rRNA genes, 16S (r rs), 23S (rrl), and 5S (rrf), are not linked in the typical arrangements fo r bacteria. Rather, the 16S rRNA gene has been separated from the 23S and 5 S rRNA gene cluster, and the 23S rRNA gene is preceded by a gene which code s for methionyl-tRNA(f)(MET) formyltransferase (fmt). In this study, we scr eened the genus Rickettsia far the fmt-rrl motif in order to examine the ph ylogenetic depth of this unusual rRNA gene organization. A rearranged opero n structure was observed in Rickettsia conorii, Rickettsia parkeri, rickett sia sibirica, rickettsia rickettsii, Rickettsia amblyomii, Rickettsia monta na, Rickettsia rhipicephali, Rickettsia australis, Rickettsia akari, Ricket tsia felis, Rickettsia canada, and Rickettsia typhi. There is also evidence for a divided operon in Rickettsia belli, but in this species, the fmt gen e could not be identified upstream of the 23S rRNA gene. In order to place the rearrangement event in the evolutionary history of the Rickettsia, phyl ogenetic analyses were performed based on the fmt-rrl spacer regions and th e 23S rRNA genes. Based on these phylogenies, we suggest that the genomic r earrangement of the rRNA genes preceded the divergence of the typhus group and the spotted fever group Rickettsia. The unique organization of the 23S rRNA genes provides a simple diagnostic tool for identification of Ricketts ia species.