The discovery of a giant virus that falls ill through infection by another virus1 is fuelling the debate about whether viruses are alive.
Before this there is more emphasis that virus are non-living as they don’t have true cellular shape and cannot replicate by themselves.
Scientists said now that there is no dout that viruses are living organims. Because if they make other sick, make it more alive.
Giant viruses have been captivating virologists since 2003, when a team led by Claverie and Didier Raoult at CNRS UMR, also in Marseilles, reported the discovery of the first monster2. The virus had been isolated more than a decade earlier in amoebae from a cooling tower in Bradford, UK, but was initially mistaken for a bacterium because of its size, and was relegated to the freezer.
Closer inspection showed the microbe to be a huge virus with, as later work revealed, a genome harbouring more than 900 protein-coding genes3 — at least three times more than that of the biggest previously known viruses and bigger than that of some bacteria. It was named Acanthamoeba polyphaga mimivirus (for mimicking microbe), and is thought to be part of a much larger family. “It was the cause of great excitement in virology,” says Eugene Koonin at the National Center for Biotechnology Information in Bethesda, Maryland. “It crossed the imaginary boundary between viruses and cellular organisms.”
Now Raoult, Koonin and their colleagues report the isolation of a new strain of giant virus from a cooling tower in Paris, which they have named mamavirus because it seemed slightly larger than mimivirus. Their electron microscopy studies also revealed a second, small virus closely associated with mamavirus that has earned the name Sputnik, after the first man-made satellite.
With just 21 genes, Sputnik is tiny compared with its mama — but insidious. When the giant mamavirus infects an amoeba, it uses its large array of genes to build a ‘viral factory’, a hub where new viral particles are made. Sputnik infects this viral factory and seems to hijack its machinery in order to replicate. The team found that cells co-infected with Sputnik produce fewer and often deformed mamavirus particles, making the virus less infective. This suggests that Sputnik is effectively a viral parasite that sickens its host — seemingly the first such example.
The team suggests that Sputnik is a ‘virophage’, much like the bacteriophage viruses that infect and sicken bacteria. “It infects this factory like a phage infects a bacterium,” Koonin says. “It’s doing what every parasite can — exploiting its host for its own replication.”
Sputnik’s genome reveals further insight into its biology. Although 13 of its genes show little similarity to any other known genes, three are closely related to mimivirus and mamavirus genes, perhaps cannibalized by the tiny virus as it packaged up particles sometime in its history. This suggests that the satellite virus could perform horizontal gene transfer between viruses — paralleling the way that bacteriophages ferry genes between bacteria.
The findings may have global implications, according to some virologists. A metagenomic study of ocean water4 has revealed an abundance of genetic sequences closely related to giant viruses, leading to a suspicion that they are a common parasite of plankton. These viruses had been missed for many years, Claverie says, because the filters used to remove bacteria screened out giant viruses as well. Raoult’s team also found genes related to Sputnik’s in an ocean-sampling data set, so this could be the first of a new, common family of viruses. “It suggests there are other representatives of this viral family out there in the environment,” Koonin says.
By regulating the growth and death of plankton, giant viruses — and satellite viruses such as Sputnik — could be having major effects on ocean nutrient cycles and climate. “These viruses could be major players in global systems,” says Curtis Suttle, an expert in marine viruses at the University of British Columbia in Vancouver.
“I think ultimately we will find a huge number of novel viruses in the ocean and other places,” Suttle says — 70% of viral genes identified in ocean surveys have never been seen before. “It emphasizes how little is known about these organisms — and I use that term deliberately.”
References:
1. La Scola, B. et al. Nature doi:10.1038/nature07218 (2008).
2. La Scola, B. et al. Science 299, 2033 (2003).
3. Raoult, D. et al. Science 306, 1344–1350 (2004).
4. Monier, A., Claverie, J.-M. & Ogata, H. Genome Biol. 9, R106 (2008).
Sputnik virophage
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Sputnik virophage
Virus classification
Group: Group I (dsDNA)
Order: Unassigned
Family: Unassigned
Genus: Unassigned
Species: Sputnik Virophage
The Sputnik virophage (from "virus" and Greek φάγειν phagein "to eat") has a functional similarity with a bacteriophage. It is an icosahedral subviral agent that is 50 nanometres in size.[1] Sputnik has been found to multiply inside of an Amoeba, although the conditions for this are rather unusual. The Subviral agent is unable to multiply itself inside of the host cell on its own, but but when the host cell has been infected sputnik harnesses the viral proteins to rapidly produce new copies of itself. Sputnik has a circular double stranded DNA genome which contains genes able to infect all three domains of life: Eukarya, Archaea and Bacteria. It is associated with the mamavirus, which presumably is related to Acanthamoeba polyphaga mimivirus (APMV). 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.[2] 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 A.polyphaga with water containing an original strain of APMV, it was discovered that several capsid layers accumulate unsymmetrically 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.[1]
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 has a high A + T 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. Considering its functional analogy with bacteriophages, this virus is classified as a virophage (ie a virus that infects other viruses).[3]
Sputnik was found to contain genes that were shared by the mimivirus. 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 the 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.[4]
Figure-1: A Sputnik virophage
References
1. ^ a b Bernard La Scola, Christelle Desnues, Isabelle Pagnier, Catherine Robert, Lina Barrassi, Ghislain Fournous, Michèle Merchat, Marie Suzan-Monti, Patrick Forterre, Eugene Koonin and Didier Raoult (2008). "The virophage as a unique parasite of the giant mimivirus". Nature 454 (7205): 100. doi:10.1038/nature07218.
2. ^ Xie, Yun. Sputnik the virophage: a virus gets a virus.ARS technica. Science Journal.
3. ^ Scola , B. et al. 2008. The virophage as a unique parasite of the giant mimivirus. Nature 455, 100–104
4. ^ http://74.6.239.67/search/cache?ei=UTF-8&p=virophage&y=Search&xa=fAH7qo6Dzk93kpVWXtGZ.A--%2C1228194443&fr=yfp-t-501&u=www.asm.org/microbe/index.asp%3Fbid%3D61386&w=virophage&d=PFqWc0fiR3Tw&icp=1&.intl=us
External links• Viralzone: Sputnik virophage
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