Molecular Basis of the Obligate Intracellular Parasitism
of Rickettsiae: A Hypothesis

V. V. Emelyanov

Gamaleya Institute of Epidemiology and Microbiology, Russian Academy of Medical Sciences, Moscow, 123098 Russia

Received May 28, 1998; in final form, January 15, 1999

Abstract—The nature of the obligate intracellular parasitism of rickettsiae, a large group of bacterial endosym-
bionts of eukaryotes, still remains an enigma. The present attempt to explain the phenomenon of rickettsial par-
asitism at the molecular level is based upon the idea about their profound similarity with mitochondria. It is
suggested that rickettsial cells incorporate from the cytoplasm of the eukaryotic cell some nuclear-encoded
mitochondrial proteins that are lacking in rickettsiae but required for their propagation. Of main importance for
the hypothesis is the evolutionary relationship of mitochondria and bacteria of family Rickettsiaceae shown in
phylogenetic studies using different molecular chronometers. Detailed phylogenetic analysis based on amino
acid sequences of the conserved heat-shock proteins Hsp60 and Hsp10 reveals the least divergence of Rickettsia
prowazekii from the last common ancestor of bacteria and mitochondria.

Key words: obligate intracellular parasitism, rickettsiae and mitochondria, heat-shock proteins, phylogenetic
analysis, evolution

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