Have you ever wondered whether the virus that gave you a nasty cold can catch one itself? It may comfort you to know that 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. Sometimes, a virus enters a cell only to find that its new temporary dwelling is already home to another dormant virus. 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.
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 and needs the genetic material of P2 to replicate and spread.
Subsequent research has shown that most bacterial species have a diverse set of satellite-helper systems. But viral satellites are not limited to bacteria. In recent years, scientists have discovered plant viral satellites that lurk in plant cells waiting for other viruses, which can have important effects on crops.
Satellites and their helpers are engaged in an endless evolutionary arms race. They evolve new ways to exploit helpers, and helpers evolve countermeasures to block them. Recent work indicates that many antiviral systems thought to have evolved in bacteria may have originated in phages and their satellites. With their high turnover and mutation rates, helper viruses and their satellites turn out to be evolutionary hot spots for antiviral weaponry.
Recently, a satellite bacteriophage called MiniFlayer was discovered. It is the first satellite phage known to have lost its ability to lie dormant. Instead, MiniFlayer has evolved a short appendage that allows it to latch onto its helper virus’s neck, similar to a vampire. Together, they search for a new host, where the viral drama will unfold again. The exact mechanism of how MiniFlayer subdues its helper and whether the helper has evolved countermeasures are still unknown.
Understanding the complex and predatory nature of viruses and their satellites can significantly impact our understanding of antiviral strategies and the development of new antiviral treatments.