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Sputnik virophage (from Russian cпутник "satellite", Latin "virus" and Greek φάγειν ''phagein'' "to eat") is a subviral agent that reproduces in amoeba cells that are already infected by a certain helper virus; Sputnik uses the helper virus's machinery for reproduction and inhibits replication of the helper virus. Viruses like Sputnik that depend on co-infection of the host cell by helper viruses are known as satellite viruses. At its discovery in a Paris water-cooling tower in 2008 Sputnik was the first known satellite virus that inhibited replication of its helper virus and thus acted as a parasite of that virus. In analogy to the term ''bacteriophage'' it was called a ''virophage''.〔 However, the usage of this term and the relationship between virophages and classical satellite viruses remain controversial. ==Virology== Sputnik was first isolated from a sample obtained from humans; it was harvested from the contact lens fluid of an individual with keratitis. Naturally however, the Sputnik virophage has been found to multiply inside species of the opportunistically pathogenic protozoan ''Acanthamoeba'' , but only if that amoeba is infected with the large mamavirus. Sputnik harnesses the mamavirus proteins to rapidly produce new copies of itself. Mamavirus is formally known as ''Acanthamoeba polyphaga'' mimivirus (APMV) and is a close relative of the previously known mimivirus. The mimivirus is a giant in the viral world; it has more genes than many bacteria and performs functions that normally occur only in cellular organisms. The mamavirus is even larger than the mimivirus, but the two are very similar in that they form large viral factories and complex viral particles. There are conditions in which Sputnik cannot produce new virions within these viruses however. It has been observed that when Mimivirus is cultured with germ-free amoeba, bald virions are produced that lack the surface fibers that are characteristic of this virus. For reasons unknown, Sputnik is unable to replicate and produce new virions in these bald viruses. Virophage growth is deleterious to APMV and results in the production of abortive forms and abnormal capsid assembly of APMV. In one of the experiments done by inoculating ''Acanthamoeba polyphaga'' with water containing an original strain of APMV, it was discovered that several capsid layers accumulate asymmetrically on one side of the viral particle causing the virus to become ineffective. Sputnik decreased the yield of infective viral particle by 70% and also reduced the amoeba lysis by threefold at 24h.〔 Sputnik has a circular double stranded DNA genome consisting of 18,343 base pairs.〔 It contains genes able to infect all three domains of life: Eukarya, Archaea and Bacteria. Of the twenty-one predicted protein-coding genes, three are apparently derived from APMV itself, one is a homologue of an archaeal virus, and four others are homologues of proteins in bacteriophages and eukaryotic viruses. Thirteen are ORFans, that is they do not have any detectable homologues in current sequence databases. The Sputnik genome has a high AT-content (73%) similar to that of APMV. Several other homologues such as those of a primase–helicase, a packaging ATPase, an insertion sequence transposase DNA-binding subunit, and a Zn-ribbon protein, were detected in the Global Ocean Survey environmental data set, suggesting that virophages could be a currently unknown family of viruses. Sputnik was found to contain genes that were shared by APMV. These genes could have been acquired by Sputnik after the association of APMV with the host and then interaction between the virophage and the viral host. Recombination within the viral factory might have resulted in the exchange of genes. Sputnik is one of the most convincing pieces of evidence for gene mixing and matching between viruses. The presence of these genes homologous to the mimivirus in Sputnik suggests that gene transfer between Sputnik and the mimivirus can occur during the infection of ''Acanthamoeba''. Therefore, it is hypothesized that the virophage could be a source of vehicle mediating lateral gene transfer between giant viruses, which constitute a significant part of the DNA virus population in marine environments. Moreover, the presence of three APMV genes in Sputnik implies that gene transfer between a virophage and a giant virus is crucial to viral evolution.〔(【引用サイトリンク】title=Biggest Known Virus Yields First-Ever Virophage )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sputnik virophage」の詳細全文を読む スポンサード リンク
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