Little more than protein capsules chock-full of genetic material, viruses barely rank among the living. Yet like people, at least one virus can catch a virus--the viral equivalent of coming down with the flu. This "flu" virus impairs the host virus's ability to grow and reproduce, a research team studying the largest known viruses reports.
Viruses are tiny biological hijackers that cause diseases that include the common cold, the flu, chickenpox, and AIDS. They infect animals, plants, and microorganisms and use their host's cellular machinery to make copies of themselves. Typically, viruses are particles so tiny that they can be easily seen only under an electron microscope. Then 5 years ago, bacteriologist Bernard La Scola and microbiologist Didier Raoult, both of the University of the Mediterranean in Marseille, France, and their colleagues identified in an amoeba a virus that was so big it was visible with a light microscope. The chromosome and size of that jumbo "Mimivirus" are on par with some bacteria, more than 10 times the size of the average viral chromosome (Science, 28 March 2003, p. 2033).
While examining a new giant amoeba virus in a cooling tower in Paris, the researchers found that the virus itself hosted tiny viral particles. They dubbed the virus's virus Sputnik and called it a "virophage" to parallel "bacteriophage," which is the name for a virus that infects bacteria. Unlike most viruses, Sputnik does not replicate by itself in amoeba and thus must hook up with the giant virus in order to persist. Infected giant viruses grow abnormally, with thicker capsules, and produce fewer progeny, La Scola and his colleagues report in the 7 August issue of Nature. They don't know if Sputnik is associated with other viruses.
Sputnik is puny: It has a circular chromosome that's just 18,000 bases long compared with the giant virus's 1.2-million-base linear chromosome. The chromosome is a mosaic of 21 protein-coding genes. Among them are three that appear to have been kidnapped from the giant virus, as well as a few that seem to have been transferred from viruses that infect bacteria or other microorganisms known as archaea. Viruses were known to pick up genes from other organisms, but this "is the first time that there has been horizontal transfer between viruses," says La Scola. Furthermore, 13 of its genes are completely new to researchers. And because some of Sputnik's genes have turned up in large-scale surveys of genetic material isolated from the ocean, La Scola expects that there are many more virophages waiting to be discovered.
"This is a truly fascinating observation," says Eddie Holmes, an evolutionary biologist at Pennsylvania State University in State College. "There is a growing realization that we are only scratching the surface of the 'virosphere,' and this is a beautiful example" of that fact. Michael Chandler, a molecular biologist from the Université Paul Sabatier in Toulouse, France, says more work needs to be done to prove Sputnik is a true virus; in particular, researchers must demonstrate that it actively infects the giant virus. But if Sputnik really is a virus, "it may ... contribute to understanding viral evolution and interkingdom genetic exchange."