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Return to Home : November 2008 : Current Topics

Biggest Known Virus Yields First-Ever Virophage

A newly discovered mammoth mimivirus is itself unexpectedly infected with a tiny icosahedral virus, according to Bernard La Scola and Didier Raoult of the Universite´ of Mediterrane ´e in Marseille, France, and their collaborators in France and the United States. Aptly, they call the new 50-nm viral parasite “Sputnik,” after “traveling companion” in Russian, while the new and much larger strain of mimivirus that carries it is formally known as an Acanthamoeba polyphaga mimivirus (APMV) or, less formally, “mamavirus.” Details appear in the 4 September 2008 (6 August online) Nature (doi:10.1038/nature07218).

The main features of mamavirus closely resemble those seen in other mimiviruses. For instance, it forms complex virus-producing factories, and the particle is surrounded by a characteristic, fibril-covered, multilayered membrane. By contrast, Sputnik is rich in novelty and its genes appear to derive from several possible sources, including an unknown family of small viruses, an archaeal virus or plasmid, and a mimivirus. “One of Sputnik’s most remarkable characteristics is its apparent chimeric origin,” La Scola says. “[It] seems to be one of the most convincing cases so far for gene mixing and matching within the virus world.”

La Scola and his collaborators spotted the smaller viruses reproducing inside mamaviruses, and transmission electron microscopy reveals that mamavirus and Sputnik particles are produced simultaneously but at different locations and rates. Importantly, Sputnik reproduction interferes with that of APMV, resulting in deformed mamavirus progeny. In most cases, several capsid layers accumulate asymmetrically at only one pole of the mamavirus particles, while fibril distribution becomes erratic. Further, when mamavirus and Sputnik are coinoculated into amoebas, the yield of infective mamavirus particles decreases by roughly 70%. Sputniks, it seems, sicken the mamavirus.

“The finding that Sputnik can pirate the factory of another virus to propagate at the expense of its host makes it a ‘virophage,’ an infecting agent functionally analogous to bacteriophage,” La Scola says. He urges a search for additional virophages “to shed more light on the unique modes of viral interactions.” For example, virophage might serve as vehicles for lateral gene transfer between giant viruses, which constitute an important part of the DNA virus population in diverse marine environments, he points out. “Indeed, the presence of three APMV genes in Sputnik suggests that gene transfer between a virophage and a giant virus is not only possible, it’s probably central to viral evolution.”

Jean-Michel Claverie, codiscoverer of the first mimivirus and director of the Mediterranean Institute of Microbiology in Marseille, considers the discovery of Sputnik a pivotal one and says “the fact that a virus can get sick makes it more alive.” According to Claverie, “discovery of a virophage infecting a mamavirus factory lends credence to the radical new view known as ‘Girus,’ which defines the factory, not the infecting particle, as the viral organism.” It also holds that viruses are alive, even though they cannot reproduce entirely on their own. Thus, Claverie and others in this camp see no difference between the cell-like virus factory and intracellular bacteria such as
Rickettsia and Chlamydia, rendering viruses as bona fide “living” microorganisms.

Marcia Stone
Marcia Stone is a science writer based in New York City. More of her work can be seen on www.mstoneworks.net.

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