The genome of Rickettsia prowazekii and some thoughts on the origin of mitochondria and hydrogenosomes

The sequence of an α-proteobacterial genome, that of Rickettsia prowazekii,⁽¹⁾ is a substantial advance in microbial and evolutionary biology. The genome of this obligately aerobic intracellular parasite is small and is apparently still undergoing reduction, reflecting gene losses attributable to its intracellular parasitic lifestyle. Evolutionary analyses of proteins encoded in the genome contain the strongest phylogenetic evidence to date for the view that mitochondria descend from α-proteobacteria. Although both Rickettsia and mitochondrial genomes are highly reduced, it appears that genome reduction in these lineages has occurred independently. Rickettsia's genome encodes an ATP-generating machinery that is strikingly similar to that of aerobic mitochondria. But it does not encode homologues for the ATP-producing pathways of anaerobic mitochondria or hydrogenosomes, leaving an important issue regarding the origin and nature of the ancestral mitochondrial symbiont unresolved. BioEssays 21:377–381, 1999. © 1999 John Wiley & Sons, Inc.