Rickettsia as ancestors of mitochondria

Broadly three major groups of hypotheses have been put forward for the possible origin of mitochondriate eukaryotes The first group postulates the endosymbiosis of an Archaea such as a Thermoplasma, methanogens, or a crenarchaeote (eocyte) with a Bacteria being a spirochaete, an H2-producing 8-Proteobacteria, or a Gram-negative bacterium, respectively. This is followed by an O2-consuming a-Proteobacteria such as a Rickettsia . The second group proposes that a Gram-positive bacterium gave rise to neomuran, the ancestor of both Archaea and current Bacteria The bacterial arm then engulfed again an O2-consum-ing a-Proteobacteria such as a Rickettsia . Both groups assume an amitochondriate eukary-ote as an intermediate . The fact that no ancestral amitochondriate eukaryote has ever been detected and that all extant amitochondriate eukaryotes have secondarily lost their mitochondria or still possess equivalents to mitochondria, led to the third group of hypotheses without an amitochondriate eukaryote as an intermediate step This group presumes the endosymbioses of an Archaea directly with an O2-consuming a-Proteobacteria such as a Rickettsia or an Archaea that is either a H2-comsuming or a H2S producing with an a-Proteobacteria that is H2 producing or H2S consuming, respectively; in the latter two cases the a-Proteobacteria are not Rickettsia-like

Despite a long-standing interest in the endosymbiotic origin of mitochondria (Altmann, 1980; Margulis, 1993; Sapp, 1994), no obligate endosymbiotic bacterium has been identified as a potential relative to mitochondria Many studies place the origin of mitochondria within the order Rickettsiales (Gupta, 1995; Lang et al ., 1999; Wu et al ., 2004; Fitzpatrick et al , 2006) or within the family Rickettsiaceae, in particular close to R. prowazekii (Anders-son et al ., 1998; Karlin and Brocchieri, 2000; Ogata et al ., 2001; Emelyanov, 2003) . Alternatives to a strict Rickettsia lineage have been proposed; for example, the free-living and nitrogen-oxidizing bacterium Paracoccus denitrificans (Rhodobacteraceae, Rhodobacterales, a-Proteobacteria) (John, 1987), the free-living photosynthetic bacterium Rhodospirillum rubrum of the family of purple nonsulfur bacteria Rhodospirillaceae (Rhodospirales, a-Proteobacteria) (Esser et al , 2004), and the infectious bacterium Holospora obtusa (Rickettsi-ales) that invades the somatic macronucleus of the ciliate Paramecium caudatum (Lang et al , 2005) The latter bacterium might have a beneficial effect on its host through the supply of biotin, Hsp70 and/or GroEL, which might increase the survival of the host at low temperatures and provide survival and increased motility at high temperatures (Fujishima et al , 2005) Cavalier-Smith argues on functional reasons that the mitochondrial ancestor should have been a photosynthetic nonsulfur purple bacterium (Cavalier-Smith, 2006) Examples of such bacteria as endosymbionts are known (Fenchel and Bernhard, 1993) It is also possible that the ultimate ancestor of Rickettsia could have been photosynthetic

A concatenated alignment of 15 mitochondrion-encoded proteins that are unlikely to have undergone any lateral gene transfer in the timeline under consideration places the mitochondria inside the order Rickettsiales (Fitzpatrick et al ., 2006) . A species tree for 72 a-Proteobacteria produced from concatenating the members of 104 well-behaved protein families anchors the mitochondrial branch within the Rickettsiales as a sister to the combined Anaplasmataceae and Rickettsiaceae and all embraced by Pelagibacter as an out-group (Williams et al , 2007) Regardless of the lineage, in none of these cases an obligate dependence of the host has been established The Rickettsia described in the two book-lice species are currently the closest obligate symbionts to mitochondria The biology of the booklice Rickettsia suggests that their obligate endosymbiotic lifestyle is a very recent acquisition

Unprecedented for intracellular bacteria but common for mitochondria, the import of a host protein has been reported for Rickettsia . Mitochondrial porin was identified in R. prowazekii by Western blot analysis (Emelyanov and Vyssokikh, 2006) . A rickettsial putative peptidase (RPP) of R. prowazekii that resembles the a- and P-subunits of the mitochon-drial processing peptidase can specifically hydrolyze basic host peptides and presequence peptides with frequent cleavage at their MPP-processing sites, showing that a Rickettsia enzyme can cleave the signal sequences of host proteins targeted to mitochondria (Kitada et al , 2007)

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