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A new giant virus discovered in the Siberian permafrost

A giant virus from a completely new genus was discovered in a Siberian permafrost sample dating back 30,000 years. Genomics, transcriptomics, proteomics and metagenomics draw a detailed portrait of this new virus, known as Mollivirus sibericum.

Published on 7 September 2015

After the discoveries of Megaviridae (2003), Pandoraviridae (2013) and Pithovirus (2014), a fourth giant virus family has been brought to light thanks to a collaboration involving the CEA-IG, the CNRS, the Université Aix-Marseille, INSERM and the Université Joseph Fourier. This team, who was already behind the discovery of Pithovirus, has isolated, amplified and characterized this new virus, Mollivirus sibericum, found in the same frozen soil sample from the extreme north-east of Siberia where Pithovirus had been detected. This is the first time that all of the techniques to analyze the living, including genomics, transcriptomics, proteomics and metagenomics, have been used simultaneously to characterize a virus.

This Mollivirus, which infects amoebas of the genus Acanthamoeba, has as an oblong shell about 0.6 μm in length that encloses a genome of about 650,000 base pairs, coding for more than 500 proteins. Most of these proteins do not have the slightest resemblance to those of its Siberian predecessor, Pithovirus sibericum. In contrast to Pithovirus, which only requires the cytoplasm resources of its host cell to multiply, Mollivirus sibericum also uses the cell nucleus to replicate in the amoeba, which makes it as dependent on its host as most "small" viruses. This strategy draws Mollivirus closer to the common types of human viral pathogens such as adenoviruses, papillomaviruses, and herpesviruses. As for Pithovirus, it multiplies in the cytoplasm in the manner of the Poxviridae, i.e. the family that includes the officially eradicated smallpox virus. With its shape, mode of replication and metabolism, Mollivirus sibericum firmly represents a new virus family distinct from the three giant virus families already identified. Indeed, this new type of virus has never been observed before.

This discovery, which suggests that giant viruses are not uncommon and are very diverse, also demonstrates that their ability to survive in the permafrost over very long periods is not limited to a particular type of virus, but probably extends across families with very different replication strategies and that are therefore potentially pathogenic. The metagenomics analysis results of this permafrost sample, which shows an extremely low concentration of Mollivirus (on the order of a few parts per million), have important implications for public health today. Several persistently infectious viral particles may indeed be sufficient, in the presence of a susceptible host, for the resurgence of potentially pathogenic viruses in Arctic regions, which are increasingly coveted for their mineral and oil resources and whose accessibility and industrial exploitation are facilitated by climate change.

Scanning electron microscopy of the particles of the 4 giant virus families currently known. The largest dimensions range from approximately 0.6 micron (Mollivirus) to 1.5 micron (Pandoravirus). © IGS CNRS/AMU.

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