Sputnik was first isolated in 2008 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 mamavirus proteins to rapidly produce new copies of itself. 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. 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. It contains 16 predicted
hairpin loops, all but two of which fall between ORFs. Several other homologues such as those of a
helicase-primase, 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. ==Structure==