The virophage – a virus that infects other viruses
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.
Metavirus
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
Go Further
Animals
- Octopuses have a lot of secrets. Can you guess 8 of them?
- Animals
- Feature
Octopuses have a lot of secrets. Can you guess 8 of them? - This biologist and her rescue dog help protect bears in the AndesThis biologist and her rescue dog help protect bears in the Andes
- An octopus invited this writer into her tank—and her secret worldAn octopus invited this writer into her tank—and her secret world
- Peace-loving bonobos are more aggressive than we thoughtPeace-loving bonobos are more aggressive than we thought
Environment
- Listen to 30 years of climate change transformed into haunting musicListen to 30 years of climate change transformed into haunting music
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- U.S. plans to clean its drinking water. What does that mean?U.S. plans to clean its drinking water. What does that mean?
- Food systems: supporting the triangle of food security, Video Story
- Paid Content
Food systems: supporting the triangle of food security - Will we ever solve the mystery of the Mima mounds?Will we ever solve the mystery of the Mima mounds?
History & Culture
- Strange clues in a Maya temple reveal a fiery political dramaStrange clues in a Maya temple reveal a fiery political drama
- How technology is revealing secrets in these ancient scrollsHow technology is revealing secrets in these ancient scrolls
- Pilgrimages aren’t just spiritual anymore. They’re a workout.Pilgrimages aren’t just spiritual anymore. They’re a workout.
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- This ancient cure was just revived in a lab. Does it work?This ancient cure was just revived in a lab. Does it work?
Science
- The unexpected health benefits of Ozempic and MounjaroThe unexpected health benefits of Ozempic and Mounjaro
- Do you have an inner monologue? Here’s what it reveals about you.Do you have an inner monologue? Here’s what it reveals about you.
- Jupiter’s volcanic moon Io has been erupting for billions of yearsJupiter’s volcanic moon Io has been erupting for billions of years
- This 80-foot-long sea monster was the killer whale of its timeThis 80-foot-long sea monster was the killer whale of its time
Travel
- How to plan an epic summer trip to a national parkHow to plan an epic summer trip to a national park
- This town is the Alps' first European Capital of CultureThis town is the Alps' first European Capital of Culture
- This royal city lies in the shadow of Kuala LumpurThis royal city lies in the shadow of Kuala Lumpur
- This author tells the story of crypto-trading Mongolian nomadsThis author tells the story of crypto-trading Mongolian nomads