title: The virophage as a unique parasite of the giant mimivirus.
abstract: 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.
authors.affiliation: URMITE, Centre National de la Recherche Scientifique UMR IRD 6236, Faculté de Médecine, Université de la Méditerranée, 27 Boulevard Jean Moulin, 13385 Marseille Cedex 5, France.
authors.forename: Bernard
authors.initials: B
authors.lastname: La Scola
authors.forename: Christelle
authors.initials: C
authors.lastname: Desnues
authors.forename: Isabelle
authors.initials: I
authors.lastname: Pagnier
authors.forename: Catherine
authors.initials: C
authors.lastname: Robert
authors.forename: Lina
authors.initials: L
authors.lastname: Barrassi
authors.forename: Ghislain
authors.initials: G
authors.lastname: Fournous
authors.forename: Michèle
authors.initials: M
authors.lastname: Merchat
authors.forename: Marie
authors.initials: M
authors.lastname: Suzan-Monti
authors.forename: Patrick
authors.initials: P
authors.lastname: Forterre
authors.forename: Eugene
authors.initials: E
authors.lastname: Koonin
authors.forename: Didier
authors.initials: D
authors.lastname: Raoult
mesh_terms: D000656:Amoeba / Q000821:virology*; D000818:Animals; D004267:DNA Viruses / Q000235:genetics / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D022761:Gene Transfer, Horizontal; D005814:Genes, Viral / Q000235:genetics; D016679:Genome, Viral / Q000235:genetics; D008969:Molecular Sequence Data; D016366:Open Reading Frames / Q000235:genetics; D010271:Parasites / Q000235:genetics / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D014764:Viral Proteins / Q000235:genetics / Q000378:metabolism
chemical_list: D014764:Viral Proteins