Mitochondria, the so-called powerhouse of the cells of animals and plants, were "energy parasites" before they became the beneficial cellular energy factories that they are today, findings of a new research suggest.

In the new study published in the journal PLOS ONE on October 15, Martin Wu, from the Department of Biology of the University of Virginia, and colleagues decoded the genomes of 18 bacteria that are closely related to mitochondria using advance DNA sequencing technologies.

Mitochondria are known to have come into scene about 2 billion years ago but its origins are not fully known. Wu said that by reconstructing the gene content of the ancestors of the mitochondria using the sequenced DNA of its close relatives, they found evidence that suggests mitochondria used to be a parasite that stole energy from its host, which is completely in contrast with the role that the membrane organelle has today.

"Our results suggest that mitochondria most likely originated from an obligate intracellular parasite and not from a free-living bacterium. This has important implications for our understanding of the origin of mitochondria," the researchers wrote. "It implies that at the beginning of the endosymbiosis, the bacterial symbiont provided no benefits whatsoever to the host.

Mitochondria serve as the power plant of the cells by supplying them with the so called "molecular unit of currency," adenosine triphosphate (ATP), a molecule known as the energy currency of life because it stores chemical energy from the breakdown of food molecules and then releases it to power cellular processes. The researchers said that contrary to the current mitochondrial function, the ancestors of the mitochondria stole ATP from their host.

Wu said that the findings of their study could change the notions of how mitochondria came about as the results of the research offer an alternative to current theories. Their study suggests that the early relatives of mitochondria were parasitic to their host and that they only became beneficial when the direction of the ATP transport was switched later on.

"We are saying that the current theories - all claiming that the relationship between the bacteria and the host cell at the very beginning of the symbiosis was mutually beneficial - are likely wrong," Wu said. "Instead, we believe the relationship likely was antagonistic - that the bacteria were parasitic and only later became beneficial to the host cell by switching the direction of the ATP transport."

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