Evolutionary relationship of Rickettsiae and mitochondria

Phylogenetic data support an origin of mitochondria from the alpha -proteobacterial order Rickettsiales, This high-rank taxon comprises exceptionally obligate intracellular endosymbionts of eukaryotic cells, and includes family Rickettsiacleae and a group of microorganisms termed Rickettsia-like endosymbionts (RLEs). Most detailed phylogenetic analyses of small subunit rRNA and chaperonin 60 sequences consistently show the RLEs to have emerged before Rickettsiaceae and mitochondria sister clades, These data suggest that the origin of mitochondria and Rickettsiae has been preceded by the long-term mutualistic relationship of an intracellular bacterium with a pro-eukaryote, in which an invader has lost many dispensable genes, yet evolved carrier proteins to exchange respiration-derived ATP for host metabolites as envisaged in classic endosymbiont theory. (C) 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.