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Redefining viruses: lessons from Mimivirus


Viruses are the most abundant living entities and probably had a major role in the evolution of life, but are still defined using negative criteria. Here, we propose to divide biological entities into two groups of organisms: ribosome-encoding organisms, which include eukaryotic, archaeal and bacterial organisms, and capsid-encoding organisms, which include viruses. Other replicons (for example, plasmids and viroids) can be termed 'orphan replicons'. Based on this suggested classification system, we propose a new definition for a virus — a capsid-encoding organism that is composed of proteins and nucleic acids, self-assembles in a nucleocapsid and uses a ribosome-encoding organism for the completion of its life cycle.

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Figure 1: Mimivirus infecting Acanthamoeba polyphaga.
Figure 2: Clusters of orthologous groups (COGs) in Mimivirus and traditional cellular organisms.
Figure 3: Capsid proteins from viruses that infect organisms from all three domains of life.
Figure 4: Redefining viruses.

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The authors thank A. Hecker and P.E. Fournier for help with the figures.

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Correspondence to Didier Raoult.

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Raoult, D., Forterre, P. Redefining viruses: lessons from Mimivirus. Nat Rev Microbiol 6, 315–319 (2008).

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