The sequence of an alpha-proteobacterial genome, that of Rickettsia prowazekii, 
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 alpha-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.