Rickettsiales

Mitochondria are believed to have developed from an endosymbiotic union with alpha-proteobacteria, specifically the Rickettsiales.

The genome of Rickettsia prowazekii is similar to mitochondrial genomes. Phylogenetic analyses indicate that R. prowazekii is more closely related to mitochondria than is any other microbe yet analyzed. Neither genome contains genes required for anaerobic glycolysis. R. prowazekii does contain a complete set of genes encoding components of the tricarboxylic acid cycle and the respiratory-chain complex, so ATP production in Rickettsia is the same as that in mitochondria. The genes from Rickettsia prowazekii encoding cytochrome b (cob) and cytochrome c oxidase subunit I (cox1) provide further phylogenetic evidence for a link with nitochondrial origins.

Rickettsia are obligate intracellular pathogens. They are dependent on invasion, growth, and replication within the cytoplasm of a eukaryotic host cell. Rickettsial progeny are released to initiate a new infection cycle when the host cell undergoes lysis. It is theorized that Rickettsiae survive and reproduce within host cells because they are capable of "suppression of the antimicrobial activities of the eukaryotic target cells, specifically monocytes/macrophages" (Radulovic et al. 2001).

A phylogenetic analysis of the cytochrome b and cytochrome c oxidase I genes supports an origin of mitochondria from within the Rickettsiaceae.
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.

Sicheritz-Ponten T, Kurland CG, Andersson SG. A phylogenetic analysis of the cytochrome b and cytochrome c oxidase I genes supports an origin of mitochondria from within the Rickettsiaceae. Biochim Biophys Acta. 1998 Jul 20;1365(3):545-51.

The genome sequence of Rickettsia prowazekii and the origin of mitochondria.
We describe here the complete genome sequence (1,111,523 base pairs) of the obligate intracellular parasite Rickettsia prowazekii, the causative agent of epidemic typhus. This genome contains 834 protein-coding genes. The functional profiles of these genes show similarities to those of mitochondrial genes: no genes required for anaerobic glycolysis are found in either R. prowazekii or mitochondrial genomes, but a complete set of genes encoding components of the tricarboxylic acid cycle and the respiratory-chain complex is found in R. prowazekii. In effect, ATP production in Rickettsia is the same as that in mitochondria. Many genes involved in the biosynthesis and regulation of biosynthesis of amino acids and nucleosides in free-living bacteria are absent from R. prowazekii and mitochondria. Such genes seem to have been replaced by homologues in the nuclear (host) genome. The R. prowazekii genome contains the highest proportion of non-coding DNA (24%) detected so far in a microbial genome. Such non-coding sequences may be degraded remnants of 'neutralized' genes that await elimination from the genome. Phylogenetic analyses indicate that R. prowazekii is more closely related to mitochondria than is any other microbe studied so far.

Andersson SG, Zomorodipour A, Andersson JO, Sicheritz-Ponten T, Alsmark UC, Podowski RM, Naslund AK, Eriksson AS, Winkler HH, Kurland CG. The genome sequence of Rickettsia prowazekii and the origin of mitochondria. Nature. 1998 Nov 12;396(6707):133-40. Comment Nature. 1998 Nov 12;396(6707):109-10.

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

Muller M, Martin W. The genome of Rickettsia prowazekii and some thoughts on the origin of mitochondria and hydrogenosomes. Bioessays. 1999 May;21(5):377-81.

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