A study led by Claudio Bandi, professor of parasitology at the University of Milano, in collaboration with the Universities of Valencia, Pavia, Sydney and Milano Bicocca, updated the reconstruction of the bacterial ancestor of the mitochondrion, and shed light on the initial steps of the symbiosis that gave rise to the eukaryotic cell. The study has been published in the December issue of Molecular Biology and Evolution.
The eukaryotic cell (i.e. the cell with a nucleus) is the fundamental structural element of animals, plants and of a number of microorganisms, from yeasts to malaria parasites. The origin of these kinds of complex cells was a major transition in the history of life on earth. Mitochondria, the 'energetic power plants' of the cell, were once bacteria living independently, before they established a symbiosis with the ancestors of the eukaryotic cells. Millions of years of living together strengthened the association between mitochondria and host cells, slowly and gradually transforming the former into an integrated part of the latter.
The study started with the sequencing of the genome of Midichloria mitochondrii. Midichloria is a bacterium that lives in symbiosis with the tick Ixodes ricinus, a parasite of high medical importance, vector of a number of infectious diseases. Midichloria is unique, as it is the only bacterium so far described that can live and multiply within the mitochondria of the host cell. Furthermore Midichloria is evolutionarily close to mitochondrion that means that it is not only a symbiont of mitochondria, but also a 'close relative' of these organelles.
In the study the genome of Midichloria was compared with the genomes of other similar bacteria, other 'relatives' of mitochondria. The researchers found two characteristics of Midichloria that are absent from all the other bacteria analyzed: the presence of the genes for the synthesis of the flagellum, a sort of microscopic tail that confers motility to bacteria, and of the genes for a cytochrome oxidase capable of working at very low oxygen concentration. The next step in the research was to reconstruct the evolutionary history of these interesting genes, in order to trace the origin of these characteristics. The result was that both the flagellum and the oxidase are ancestral, thus present in the bacterium that established the symbiosis with the ancestor of the eukaryotic cell, to become the mitochondrion.
The conclusions of the study allow depicting a novel reconstruction of the free-living ancestor of mitochondria: it was a flagellated bacterium that thrived even under low levels of oxygen. This reconstruction clarifies some issues on the origin of eukaryotic cells that are still debated. The presence of the flagellum allows us to address questions on the mechanism of entrance of the mitochondrial ancestor into the host cell. We can imagine a mechanism analogous to that of the entrance of present-day unicellular parasites, where the flagellum plays an active, propulsive role in the entrance into the host cells. The capacity of surviving and possibly of multiplying at low oxygen concentration addresses the issue of the environment where the interaction between the ancestor of the mitochondrion and the ancestor of the eukaryotic cell was first established. If the former lived in an oxygen-rich environment (as so far commonly believed), while oxygen was toxic to the latter, how could the two partners have met? The mitochondrial ancestor, as described in this study, was likely capable of being metabolically active even at low oxygen concentrations, in micro-environments that where not prohibitive to the proto-eukaryotic cells.
Reference
Davide Sassera, Nathan Lo, Sara Epis, Giuseppe D'Auria, Matteo Montagna, Francesco Comandatore, David Horner, Juli Peretó, Alberto Maria Luciano, Federica Franciosi, Emanuele Ferri, Elena Crotti, Chiara Bazzocchi, Daniele Daffonchio, Luciano Sacchi, Andres Moya, Amparo Latorre, and Claudio Bandi “Phylogenomic Evidence for the Presence of a Flagellum and cbb3 Oxidase in the Free-Living Mitochondrial Ancestor” Mol Biol Evol (2011) 28(12): 3285-3296 doi:10.1093/molbev/msr159
Wiki Articles
http://en.wikipedia.org/wiki/Midichloria
http://en.wikipedia.org/wiki/Mitochondrion
Last update: 16 de december de 2011 07:50.
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