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The virophage as a unique parasite of the giant mimivirus


Viruses are obligate parasites of Eukarya, Archaea and Bacteria. Acanthamoeba polyphaga mimivirus (APMV) is the largest known virus; it grows only in amoeba and is visible under the optical microscope. Mimivirus possesses a 1,185-kilobase double-stranded linear chromosome whose coding capacity is greater than that of numerous bacteria and archaea1,2,3. Here we describe an icosahedral small virus, Sputnik, 50 nm in size, found associated with a new strain of APMV. Sputnik cannot multiply in Acanthamoeba castellanii but grows rapidly, after an eclipse phase, in the giant virus factory found in amoebae co-infected with APMV4. Sputnik growth is deleterious to APMV and results in the production of abortive forms and abnormal capsid assembly of the host virus. The Sputnik genome is an 18.343-kilobase circular double-stranded DNA and contains genes that are linked to viruses infecting each of the three domains of life Eukarya, Archaea and Bacteria. Of the 21 predicted protein-coding genes, eight encode proteins with detectable homologues, including three proteins apparently derived from APMV, a homologue of an archaeal virus integrase, a predicted primase–helicase, a packaging ATPase with homologues in bacteriophages and eukaryotic viruses, a distant homologue of bacterial insertion sequence transposase DNA-binding subunit, and a Zn-ribbon protein. The closest homologues of the last four of these proteins were detected in the Global Ocean Survey environmental data set5, suggesting that Sputnik represents a currently unknown family of viruses. Considering its functional analogy with bacteriophages, we classify this virus as a virophage. The virophage could be a vehicle mediating lateral gene transfer between giant viruses.

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Figure 1: Different morphological aspects of mamavirus and Sputnik.
Figure 2: Sputnik propagation in mamavirus-infected amoebae.
Figure 3: The Sputnik chromosome.

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The virophage genome has been deposited in GenBank under accession number EU606015. The Acanthamoeba castellanii mamavirus genes with homologues found in the Sputnik genome have been deposited in GenBank under accession numbers EU827539, EU827540 and EU827541.


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We thank X. de Lamballerie, S. Azza, P. de Clocquement, L. Espinosa, B. Campagna, N. Aldrovandi, V. Brice, A. Bernard, C. Ivars, B. Giumelli and Y. Wolf for expert assistance. This work was funded by the Centre National de la Recherche Scientifique (CNRS, crédits récurrents). I.P. is funded by a CIFFRE fellowship, E.K. is supported by the Intramural Research Program of the National Institutes of Health, National Library of Medicine, and P.F. is funded by the Institut Universitaire de France.

Author contributions D.R. and B.L.S. supervised the project and wrote the manuscript. C.D., P.F. and E.K. contributed to sequence analysis, interpretation of the results and writing of the manuscript. I.P. isolated the virus. M.S.-M. contributed to viral cycle analysis, interpretation of the results and writing of the manuscript. M.M. provided water samples. L.B. conducted the viral cycle experiment. C.R. and G.F. sequenced the genome.

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Correspondence to Didier Raoult.

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La Scola, B., Desnues, C., Pagnier, I. et al. The virophage as a unique parasite of the giant mimivirus. Nature 455, 100–104 (2008).

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