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New Giant Viruses Discovered, Possible Missing Link between Viruses and Cells

With the discovery of Mimivirus ten years ago and, more recently, Megavirus chilensis, researchers thought they had reached the farthest corners of the viral world in terms of size and genetic complexity. With a diameter in the region of a micrometer and a genome incorporating more than 1,100 genes, these giant viruses, which infect amoebas of the Acanthamoeba genus, had already largely encroached on areas previously thought to be the exclusive domain of bacteria. For the sake of comparison, common viruses such as the influenza or HIV viruses, only contain approximately ten genes each. In an article published in the July 19, 2013 issue of Science, the researchers from France and Sweden announced they had discovered two new giant viruses: Pandoravirus salinus, on the coast of Chile, and Pandoravirus dulcis, in a freshwater pond in Melbourne, Australia. Detailed analysis has shown that these first two Pandoraviruses have virtually nothing in common with previously characterized giant viruses. What's more, only a very small percentage (6%) of the proteins encoded by Pandoravirus salinus is similar to those already identified in other viruses or cellular organisms. With a genome of this size, Pandoravirus salinus demonstrates that viruses can be more complex than some eukaryotic cells. Another unusual feature of Pandoraviruses is that they have no gene allowing them to build a protein like the capsid protein, which is the basic building block of traditional viruses. Despite all these novel properties, Pandoraviruses display the essential characteristics of other viruses in that they contain no ribosomes, produce no energy, and do not divide. This groundbreaking research included an analysis of the Pandoravirus salinus proteome, which proved that the proteins making it up are consistent with those predicted by the virus’ genome sequence. Pandoraviruses thus use the universal genetic code shared by all living organisms on the planet. This shows just how much more there is to learn regarding microscopic biodiversity as soon as new environments are considered. The simultaneous discovery of two specimens of this new virus family in sediments located 15,000 km apart indicates that Pandoraviruses, which were completely unknown until now, are very likely not rare. These new giant viruses definitively bridge the gap between viruses and cells – a gap that was proclaimed as dogma at the very outset of modern virology in the 1950s. The discoveries also suggests that cell life could have emerged with a far greater variety of pre-cellular forms than those conventionally considered, as the new giant viruses have almost no equivalent among the three recognized domains of cellular life, namely eukaryota (or eukaryotes), eubacteria, and archaea. The image shows a 0.2 µm Pandoravirus salinus observed under the electron microscope. (Image courtesy of IGS CNRS-AMU). [Press release] [Science abstract]