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

A briefly argued case that mitochondria and plastids are descendants of endosymbionts, but that the nuclear compartment is not

“…It has been pointed out that methanogens are the only prokaryotes known that possess true histones, lending credence to the view that the host, in principle, could have been a methanogen Sandman et al 1998;Vellai et al 1998). In agreement with the view that the ancestor of mitochondria was a facultative anaerobe is, for example, the ¢nding that some eukaryotes possess a nuclear-encoded, mitochondrialtargeted homologue of the TspO protein, which in the -proteobacterium Rhodobacter regulates the switch from aerobic to anaerobic metabolism (Yeliseev et al 1997), in addition to data summarized elsewhere (Embley & Martin 1998;Martin & MÏller 1998;MÏller & Martin 1999). In addition, Chistoserdova et al (1998) reported evidence for an ancient gene transfer between methanogens and -proteobacteria, suggesting that the type of ecological, H 2 -mediated association posited might have entailed gene transfers in the other direction (from host to symbiont) as well.…”

“…The study of organelle genomes has solidly borne out that prediction (Lang et al 1997;Unseld et al 1997;Andersson et al 1998;Gray et al 1998;, indicating that these compartments do descend from endosymbionts, but, particularly in the case of mitochondria, not necessarily what kind of endosymbiont (Martin & MÏller 1998;Gray et al 1999;MÏller & Martin 1999). Similar predictions have been borne out for only two other types of cell compartments: complex plastids and hydrogenosomes.…”

“…In brief, it was suggested that the symbiont was a facultatively anaerobic -proteobacterium with considerable metabolic £exibility that was able to respire under aerobic conditions, as in classical formulations of the endosymbiont hypothesis (Doolittle 1998a), but was also able to synthesize ATP in the absence of oxygen as well, using such anaerobic pathways as are common among free-living proteobacteria. Such pathways include, for example, the H 2 -producing fermentations found in hydrogenosomes (MÏller 1993(MÏller , 1998Akhmanova et al 1998) or the ATP-producing pathways found in anaerobic mitochondria (Kobayashi & Shoun 1995;Kobayashi et al 1996;Embley & Martin 1998;MÏller & Martin 1999).…”

The reducible complexity of a mitochondrial molecular machine

“…Species of ␣-proteobacteria, the bacterial group from which mitochondria evolved (7)(8)(9), have proteins with sequence simi- larity to the mitochondrial Tim44 (Fig. 1 A) (10).…”

“…Multiple lines of evidence show that the mitochondria in our cells evolved from intracellular bacteria (9)(10)(11), and that conversion of these intracellular bacteria into mitochondria required the evolution of protein transport machines (12). We proposed that simple ''core'' machines were established in the first eukaryotes by drawing on preexisting bacterial proteins that had previously provided distinct functions.…”