The eubacterial genus belongs to the α subgroup of the phylum . This genus is usually divided into three biotypes on the basis of vector host and antigenic cross-reactivity characteristics. However, the species does not fit into this classification scheme; this organism has characteristics common to both the spotted fever group and the typhus group biotypes and also exhibits some unique features. Sequences of the 16S rRNA and 23S rRNA genes from (spotted fever group), (typhus group), and were studied to determine the position of in the rickettsial classification scheme. The 23S rRNA gene sequences described in this paper are the first 23S rRNA sequences reported for any member of the . The 23S rRNA gene contains substantially more phylogenetic information than is contained in the 16S rRNA sequences, and the 23S rRNA gene sequence has diverged about 1.9 times faster in the three species which we studied. Taken together, the molecular data obtained from the two genes indicate that is not a member of either the spotted fever group or the typhus group; rather, this organism appears to be the product of a divergence which predates the separation of the genus into the spotted fever group and the typhus group. Consequently, different combinations of the ancestral characteristics retained by have been retained in the more derived lineages of the genus. A comparison of the 16S rRNA and 23S rRNA gene sequences of strains with other proteobacterial sequences confirmed that the genus is a unique deeply branching member of the α subgroup of the and that the species form a monophyletic cluster. While divergence of the contemporary members of the genus occurred recently, the unique evolutionary line represented by this genus appears to be very old.


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