Viruses may cause disease but some can fall ill themselves. For the first time, a group of scientists have discovered a virus that targets other viruses. This new virus-of-viruses was discovered by Bernard La Scola and Christelle Desnues at the University of the Mediterranean, who have playfully named it Sputnik, after the Russian for "fellow traveller". It is so unique that they have classified it in an entirely new family - the "virophages" - in honour of the similarities it shares with the bacteriophage viruses that use bacteria as hosts.
The story of Sputnik started in 1992 with some dirty English water. A group of scientists were studying an amoeba taken from a cooling tower in Bradford, England, when they discovered a microscopic giant - a virus so large that it was originally mistaken for bacterium. It was only in 2003 that La Scola and colleagues conclusively showed that the new find was indeed a virus. But what a virus - APMV, or 'mimivirus', measures a whopping 400 nanometres across.
The search for giant viruses continued. La Scola's team identified another strain of APMV by inoculating the same species of amoeba with water taken from another cooling tower, this time from Paris. The new specimen seemed to eclipse even the original giant in size, and the researchers decided to call it 'mamavirus'.
When this record-breaker infects amoebae, it forms gigantic viral factories that pump out new copies of itself. When the team looked at these under an electron microscope, they found the equivalent of microscopic Russian dolls - tiny viral particles, just 50 nanometes in size and distinct from mamavirus itself. It's all very meta, and to the researchers, the fact that mamaviruses can "get sick" themselves is further evidence that viruses are indeed living things.
La Scola and Desnues found that Sputnik couldn't multiply within the amoeba by itself; it could only spread within cells that had also been infected with mamavirus. But Sputnik is no partner - by hijacking the mamavirus's replication machinery, it spreads at the expense of its larger host and substantially hinders its reproduction. In the presence of the tiny intruder, mamavirus particles assemble abnormally and surround themselves with unusually thick outer coats. As a result, their ability to infect the amoeba fell by 70%.
The virophage name is perhaps a bit misleading. Bacteriophages reproduce within the cells of bacteria, whereas Sputnik is a satellite virus, in more than name only. Like hepatitis D, it depends on another virus coinfecting a host in order to spread. But it's the fact that it does so at the expense of the mamavirus that makes it a true parasite.
In comparison to its sizeable host, Sputnik is tiny and sports a genome that is almost a hundred times smaller. Its 18,000 base-pairs of DNA contains just 21 genes and when La Scola and Desnues analysed these, they found that Sputnik is a genetic chimera - a mish-mash of different genes from different sources. Thirteen of these have no equivalent in any other known virus, while the remainder have similarities to genes from other viruses, bacteria and even more complex cells.
Three of these are closely related to mamavirus genes, suggesting that this tiniest of parasites has been raiding genetic material from its host and from other viruses. La Scola and Desnues even suggest that Sputnik could be acting as a genetic mule, shuffling genes between giant viruses. It could even explain why mimivirus has mysteriously and recently picked up bacterial genes of unknown origin.
Reference: Nature doi:10.1038/nature07218
Images: courtesy of Nature
I've heard of virus-eating viruses before. Except we always called them virions. I was actually having a discussion about them with my supervisor the other day.
btw, do you know what the 'virophage' coat is made of? It was the one thing we couldn't work out. I'm guessing proteins, but anything slightly more accurate would be great.
Why do I get an error message from DOI when I try to find this article?
Very cool. Could this have any implications for HIV research?
Lab Rat and Tom S- sent you the paper; don't know why the DOI doesn't work. It's definitely right. Latest issue of Nature if anyone's interested.
Erin - I don't know. It's probably way too early to say if how common virophages are or whether they may have a future in medicine. The authors say as much elsewhere. From New Scientist:
"It's too early to say we could use Sputnik as a weapon against big viruses or to modify them," says co-author Bernard La Scola, also at the University of the Mediterranean. "But phages are used to modify bacteria, so why not?"
Also, I've just realised that I should have opened this post with the following poem from Swift:
So, naturalists observe, a flea
Hath smaller fleas that on him prey,
And these have smaller fleas to bite em,
And so proceed, ad infinitum.
The doi doesn't work for me, either. The latest issue of Nature (Aug 7, vol. 454 no. 7205) contains a News article by Helen Pearson, but not the article. The original article is in the advance online publication section.
the thing is, can we use the virophage to kill the HIV ?
I doubt this will be useful against HIV etc., simply because HIV is a normal-sized virus. This isn't just about "meta", it's also about ecological niches.
My bet is that the "mamavirus" can be infected, specifically because it's big enough to have room for the freeloaders. Which is not to say that we couldn't experiment with "poison plasmids" that could disrupt the development of HIV et al.... but the thing is, that's a pretty dangerous game, and not necessarily "worth the candle".