Viruses That Parasitize Viruses?

Welcome to the world of virophage--viruses that parasitize other viruses. What's interesting is that the 'host' viruses are the megaviruses which are visible with light microscopy and which have genomes that can be larger than some bacteria. From Science:

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.

And these virophages might be quite common:

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.

The Nature article is available here (subscription needed).

Cited article: Bernard La Scola, Christelle Desnues, Isabelle Pagnier, Catherine Robert, Lina Barrassi, Ghislain Fournous, Michèle Merchat, Marie Suzan-Monti, Patrick Forterre, Eugene Koonin & Didier Raoult. 2008. The virophage as a unique parasite of the giant mimivirus. Nature doi:10.1038/nature07218.