We have cloned and sequenced the genes encoding cytochrome b (cob) and 
cytochrome c oxidase subunit I (cox1) from Rickettsia prowazekii, a member of 
the alpha-proteobacteria. The phylogenetic analysis supports the hypothesis that 
mitochondria are derived from the alpha-proteobacteria and more specifically 
from within the Rickettsiaceae. We have estimated that the common ancestor of 
mitochondria and Rickettsiaceae dates back to more than 1500 million years ago.