

It's not only humans who suffer from the depredations of viruses. Viruses do, too.

If that seems surprising, no wonder: until a team of French researchers watched one virus invade another, hijacking its genetic machinery and making copies of its victim's DNA, scientists didn't even know this was possible.

At this early stage, the importance of viral parasites to human health or the environment is unknown. At the very least, it expands our idea of what is possible in the viral world.

"It's not the first time such a satellite pair was discovered, but this host-parasite pair is unique," said Eugene Koonin, a virologist at the National Center for Biotechnology Information and co-author of the findings, which will be published in Nature. "It's the first time that these viruses have actually exchanged genes."

The players in this debut drama, entitled "The virophage as a unique parasite of the giant mimivirus," are stories unto themselves.

Playing the traditional role is a miniscule virus composed of just 21 genes and dubbed Sputnik – a term best-known for the Russian spacecraft, and literally translated as "satellite."

Sputnik's victim is one of a family of physically immense viruses, the first of which was isolated from a British cooling tower by Koonin's colleagues in 2003. That virus contained more than 900 genes – three times as many as any other virus. The researchers called it a mimivirus.

The virus described in the latest study was found in a Parisian cooling tower, and is even larger. They called it a mamavirus.

Sputnik, said Koonin, probably first evolved to infect bacteria, as is the tendency of other viruses. But for some unknown reason, it evolved to invade the mamavirus in which his team found it.

Once inside a mamavirus, Sputnik hijacks its genetic machinery. It uses these to make copies of the mamavirus' genes, which it adds to its own.

While this happens, the mamavirus has difficulty replicating: It is sick.

Some of Sputnik's genes appear to come from an unknown viral family that's only distantly related to already-identified viruses. Similar genes were found in oceanic water samples taken by the J. Craig

Venter Institute. Because Sputnik and its relatives may pass genes as readily as they steal them, University of British Columbia marine virologist

Curtis Suttle told Nature that they "could be major players in global systems."

Koonin agreed that this was possible. However, he cautioned, "the emphasis should be on could." Almost any statement about the role of parasitic viruses is speculation.

Bernard La Scola, a University of the Mediterranean virologist and co-author of the paper, speculated to New Scientist that Sputnik-like viruses could be used to target viruses that infect humans.

Koonin disagreed, noting that human-relevant viruses are too small to support parasites, and have been exhaustively studied without finding evidence of parasitism.

There is, alas, no flu season for the flu.

The virophage as a unique parasite of the giant mimivirus [Nature]

Images: Nature

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