The proto-mitochondrion is the hypothetical ancestral bacterial

endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...

from which all

mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...

eukaryote The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

s are thought to descend, after an episode of

symbiogenesis Symbiogenesis (endosymbiotic theory, or serial endosymbiotic theory) is the leading evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms. The theory holds that mitochondria, plastids such as chloroplasts, and possibl ...

which created the

aerobic Aerobic means "requiring air," in which "air" usually means oxygen. Aerobic may also refer to * Aerobic exercise, prolonged exercise of moderate intensity * Aerobics, a form of aerobic exercise * Aerobic respiration, the aerobic process of cellu ...

eukaryotes.

Phylogeny

The phylogenetic analyses of the few genes that are still encoded in the

genome A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

s of modern

suggest an

alphaproteobacteria ''Alphaproteobacteria'' or ''α-proteobacteria'', also called ''α-Purple bacteria'' in earlier literature, is a class of bacteria in the phylum '' Pseudomonadota'' (formerly "Proteobacteria"). The '' Magnetococcales'' and '' Mariprofundales'' ar ...

l origin for this

, in an ancient episode of

early in the history of the

s. Although the order ''

Rickettsiales The Rickettsiales, informally called rickettsias, are an order of small Alphaproteobacteria. They are obligate intracellular parasites, and some are notable pathogens, including '' Rickettsia'', which causes a variety of diseases in humans, and ...

'' has been proposed as the alphaproteobacterial sister-group of

, no definitive evidence pinpoints the alphaproteobacterial group from which the proto-mitochondrion emerged, and some contradictory evidence, especially in the early, sparse genome samplings. Martijn ''et al'' found mitochondria are a possible sister-group to all other alphaproteobacteria. The phylogenetic tree of the Rickettsidae has been inferred by Ferla ''et al.'' from the comparison of 16S + 23S ribosomal RNA sequences. Geiger ''et alii'' (2023) propose placing the recently discovered (2016) genus '' Iodidimonas –'' found in a sister clade to ''Rickettsidae'': the ''

Caulobacteridae ''Alphaproteobacteria'' or ''α-proteobacteria'', also called ''α-Purple bacteria'' in earlier literature, is a class of bacteria in the phylum ''Pseudomonadota'' (formerly "Proteobacteria"). The ''Magnetococcales'' and '' Mariprofundales'' are ...

'' – as the closest free-living relative of mitochondria, as it possesses more metabolic products matching that of mitochondria today, such as cardiolipins and

sphingolipids Sphingolipids are a class of lipids containing a backbone of sphingoid bases, which are a set of aliphatic amino alcohols that includes sphingosine. They were discovered in brain extracts in the 1870s and were named after the mythological sphi ...

, and important genetic markers such as the

COX Cox or COX may refer to: Companies * Cox Enterprises, a media and communications company ** Cox Communications, cable provider ** Cox Media Group, a company that owns television and radio stations ** Cox Automotive, an Atlanta-based busines ...

operon In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...

and the bc₁ complex (Complex IV and Complex III in mitochondria, respectively).

Metabolism

reconstructed the

proteome A proteome is the entire set of proteins that is, or can be, expressed by a genome, cell, tissue, or organism at a certain time. It is the set of expressed proteins in a given type of cell or organism, at a given time, under defined conditions. P ...

(the entire set of proteins expressed by a

) and corresponding metabolism of the proto-mitochondrion by comparing extant alpha-proteobacterial and

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

genomes. They concluded that this organism was an

alpha-proteobacterium respiring

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

glycerol Glycerol () is a simple triol compound. It is a colorless, odorless, sweet-tasting, viscous liquid. The glycerol backbone is found in lipids known as glycerides. It is also widely used as a sweetener in the food industry and as a humectant in pha ...

and other compounds provided by the host. At least 630

gene families A gene family is a set of several similar genes, formed by duplication of a single original gene, and generally with similar biochemical functions. One such family are the genes for human hemoglobin subunits; the ten genes are in two clusters on ...

derived from this organism can still be found in the 9 eukaryotic genomes analyzed in the study.

References

{{Taxonbar, from=Q7251873 Mitochondria Pseudomonadota Symbiosis

Phylogeny

Metabolism

See also

References