[Evolutionary relationship of Rickettsia and eukaryotic mitochondria]

To clarify the evolutionary relationship of rickettsiae and mitochondria, the conserved flat heat-shock protein Hsp60 was phylogenetically studied in detail by using PHYLIP and PROTML packages. The ample data set (50 species) included as many as possible representatives from the Rickettsiaceae family, mitochondrial-type homologs from Archezoa and mitochondrial homologs from Protozoa. Rickettsia prowazekii (that is the genus Rickettsia) was shown to be the least diverging member within Rickettsia--a sister group to the monophyletic clade of mitochondria. These findings were also evidenced by the phylogenetic analysis of 16S rRNA. Rickettsia-like endosymbionts (the parasites Paramecium caudatum and the etiological agent of hepatopancreatitis in shrimps) included within the order Rickettsiales appear to have diverged prior to the Rickettsiaceae/mitochondria cluster. Thus, the Rickettsiales does not seem to be a monophyletic group. An idea concerning the nature of obligate intracellular parasitism of rickettsiae is proposed in the paper from the suggested profound similarity of Rickettsiae genus bacteria and mitochondria which could have a common evolutionary history.