Have you ever wondered whether the virus that gave you a nasty cold can catch one itself? It may comfort you to know that, yes, viruses can actually get sick. Even better, as karmic justice would have it, the culprits turn out to be other viruses.
Viruses can get sick in the sense that their normal function is impaired. When a virus enters a cell, it can either go dormant or start replicating right away. When replicating, the virus essentially commandeers the molecular factory of the cell to make lots of copies of itself, then breaks out of the cell to set the new copies free.
Sometimes a virus enters a cell only to find that its new temporary dwelling is already home to another dormant virus. Surprise, surprise. What follows is a battle for control of the cell that can be won by either party.
But sometimes a virus will enter a cell to find a particularly nasty shock: a viral tenant waiting specifically to prey on the incoming virus.
I am a bioinformatician, and my laboratory studies the evolution of viruses. We frequently run into “viruses of viruses,” but we recently discovered something new: a virus that latches onto the neck of another virus.
A world of satellites
Biologists have known of the existence of viruses that prey on other viruses – referred to as viral “satellites” – for decades. In 1973, researchers studying bacteriophage P2, a virus that infects the gut bacterium Escherichia coli, found that this infection sometimes led to two different types of viruses emerging from the cell: phage P2 and phage P4.
Bacteriophage P4 is a temperate virus, meaning it can integrate into the chromosome of its host cell and lie dormant. When P2 infects a cell already harboring P4, the latent P4 quickly wakes up and uses the genetic instructions of P2 to make hundreds of its own small viral particles. The unsuspecting P2 is lucky to replicate a few times, if at all. In this case, biologists refer to P2 as a “helper” virus, because the satellite P4 needs P2’s genetic material to replicate and spread.
Bacteriophages are viruses that infect bacteria.
Subsequent research has shown that most bacterial species have a diverse set of satellite-helper systems, like that of P4-P2. But viral satellites are not limited to bacteria. Shortly after the largest known virus, mimivirus, was discovered in 2003, scientists also found its satellite, which they named Sputnik. Plant viral satellites that lurk in plant cells waiting for other viruses are also widespread and can have important effects on crops.
Viral arms race
Although researchers have found satellite-helper viral systems in pretty much every domain of life, their importance to biology remains underappreciated. Most obviously, viral satellites have a direct impact on their “helper” viruses, typically maiming them but sometimes making them more efficient killers. Yet that is probably the least of…
