‘Virophage’ suggests viruses are alive

“There’s no doubt this is a living organism,” says Jean-Michel Claverie, a virologist at the the CNRS UPR laboratories in Marseilles, part of France’s basic-research agency. “The fact that it can get sick makes it more alive.”

Evidence of illness enhances case for life.

Giant mamavirus particles (red) and satellite viruses
of mamavirus called Sputnik (green).

The discovery of a giant virus that falls ill through infection by another virus1 is fuelling the debate about whether viruses are alive.

“There’s no doubt this is a living organism,” says Jean-Michel Claverie, a virologist at the the CNRS UPR laboratories in Marseilles, part of France’s basic-research agency. “The fact that it can get sick makes it more alive.”

Giant viruses have been captivating virologists since 2003, when a team led by Claverie and Didier Raoult at CNRS UMR, also in Marseilles, reported the discovery of the first monster2. The virus had been isolated more than a decade earlier in amoebae from a cooling tower in Bradford, UK, but was initially mistaken for a bacterium because of its size, and was relegated to the freezer.

Closer inspection showed the microbe to be a huge virus with, as later work revealed, a genome harbouring more than 900 protein-coding genes3 — at least three times more than that of the biggest previously known viruses and bigger than that of some bacteria. It was named Acanthamoeba polyphaga mimivirus (for mimicking microbe), and is thought to be part of a much larger family. “It was the cause of great excitement in virology,” says Eugene Koonin at the National Center for Biotechnology Information in Bethesda, Maryland. “It crossed the imaginary boundary between viruses and cellular organisms.”

Now Raoult, Koonin and their colleagues report the isolation of a new strain of giant virus from a cooling tower in Paris, which they have named mamavirus because it seemed slightly larger than mimivirus. Their electron microscopy studies also revealed a second, small virus closely associated with mamavirus that has earned the name Sputnik, after the first man-made satellite.

With just 21 genes, Sputnik is tiny compared with its mama — but insidious. When the giant mamavirus infects an amoeba, it uses its large array of genes to build a ‘viral factory’, a hub where new viral particles are made. Sputnik infects this viral factory and seems to hijack its machinery in order to replicate. The team found that cells co-infected with Sputnik produce fewer and often deformed mamavirus particles, making the virus less infective. This suggests that Sputnik is effectively a viral parasite that sickens its host — seemingly the first such example.

The team suggests that Sputnik is a ‘virophage’, much like the bacteriophage viruses that infect and sicken bacteria. “It infects this factory like a phage infects a bacterium,” Koonin says. “It’s doing what every parasite can — exploiting its host for its own replication.”

Sputnik’s genome reveals further insight into its biology. Although 13 of its genes show little similarity to any other known genes, three are closely related to mimivirus and mamavirus genes, perhaps cannibalized by the tiny virus as it packaged up particles sometime in its history. This suggests that the satellite virus could perform horizontal gene transfer between viruses — paralleling the way that bacteriophages ferry genes between bacteria.

Virophages may be common in plankton blooms.

The findings may have global implications, according to some virologists. A metagenomic study of ocean water4 has revealed an abundance of genetic sequences closely related to giant viruses, leading to a suspicion that they are a common parasite of plankton. These viruses had been missed for many years, Claverie says, because the filters used to remove bacteria screened out giant viruses as well. Raoult’s team also found genes related to Sputnik’s in an ocean-sampling data set, so this could be the first of a new, common family of viruses. “It suggests there are other representatives of this viral family out there in the environment,” Koonin says.

By regulating the growth and death of plankton, giant viruses — and satellite viruses such as Sputnik — could be having major effects on ocean nutrient cycles and climate. “These viruses could be major players in global systems,” says Curtis Suttle, an expert in marine viruses at the University of British Columbia in Vancouver.

“I think ultimately we will find a huge number of novel viruses in the ocean and other places,” Suttle says — 70% of viral genes identified in ocean surveys have never been seen before. “It emphasizes how little is known about these organisms — and I use that term deliberately.”

Helen Pearson.

Source : Nature


1 La Scola, B. et al. Nature doi:10.1038/nature07218 (2008).

2 La Scola, B. et al. Science 299, 2033 (2003).

3 Raoult, D. et al. Science 306, 1344–1350 (2004).

4 Monier, A., Claverie, J.-M. & Ogata, H. Genome Biol. 9, R106 (2008).

One thought on “‘Virophage’ suggests viruses are alive”

  1. A strange life form has been identified in Bradford.

    Genetic analysis reveals that the organism is so bizarre and unlike anything else seen by scientists that perhaps it should be placed in its own category of living things.

    The creature, first discovered in a small industrial cooling tower on the outskirts of the city, could qualify for a new “domain” in the tree of life – where a domain is a bigger category than a kingdom or a phylum.

    The “giant virus”, dubbed the Mimivirus, or “mimicking microbe”, because it was first mistaken for a bacterium, inhabits amoebae and is more than twice as big as any other virus so far found. At about half a millionth of a metre across – around the size of a small bacterium – it is one of the few that can be seen under a light microscope.

    Two research teams in the Marseille School of Medicine, led by Prof Didier Raoult and Prof Jean-Michel Claverie, have “read” the genetic code of the organism and found a number of genes previously thought to belong only to more complex life forms.

    The size and complexity of the Mimivirus genetic code – which is 1.2 million “letters” long, at least 10 times larger than the code of a typical virus – “challenges the established frontier between viruses and parasitic cellular organisms”, they report today in the journal Science.

    One of the defining characteristics of a virus is that it is unable to make proteins independently, instead relying on the cells it infects to manufacture its proteins and thus reproduce. But the Mimivirus contains a number of genes for protein translation.

    It also contains genes for DNA repair enzymes and other proteins, all typically thought to be trademarks of cellular organisms.

    The Mimivirus – which so far has only been found in Bradford – appears to represent a new family of “nucleocytoplasmic” large DNA viruses that emerged with the first life on Earth some four billion years ago, said Prof Raoult. After much debate among his team, “for the first time we have enough genetic information to conclude that there is a fourth domain of life”, he said. “If this is true, this is revolutionary.”

    The other three domains of life are the eukaryotes, which have cells that contain a nucleus, and the prokaryotes, unicellular organisms that are divided into the bacteria and archaea.

    The family tree drawn up by Prof Claverie shows that the Mimivirus is no more related to the eukaryotes as it is to bacteria. ‘This organism is as old as all the rest of living organisms,” he said.

    However, Dr Dave Roberts, head of microbiology at the Natural History Museum, London, was “deeply sceptical” that the Mimivirus deserves to be placed in its own domain, though he agreed that it did mark a new family.

    “There are a lots of odd things turning up in the microbial world all the time,” he said.

    “It is a fascinating paper and very exciting. The virus seems to link to a group prior to the appearance of the three domains we currently recognise. But we are not convinced that the tree of life is still a branching structure when you get that deep.”

    The giant virus has not so far been linked with disease.


    Liked by 1 person

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