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The revival of such an ancestral amoeba-infecting virus used as a safe indicator of the possible presence of pathogenic DNA viruses, suggests that the thawing of permafrost either from global warming or industrial exploitation of circumpolar regions might not be exempt from future threats to human or animal health.
revealed the existence of giant DNA viruses with particles large enough to be visible under a light microscope (1, 2).
In contrast, the recently discovered Pandoraviruses exhibit larger amphora-shaped virions 1 μm in length and guanine–cytosine-rich genomes up to 2.8 Mb long encoding up to 2,500 proteins. Whereas the Megaviridae share some general features with the previously described icosahedral large DNA viruses, the Pandoraviruses appear unrelated to them.
Here we report the discovery of a third type of giant virus combining an even larger pandoravirus-like particle 1.5 μm in length with a surprisingly smaller 600 kb AT-rich genome, a gene content more similar to Iridoviruses and Marseillevirus, and a fully cytoplasmic replication reminiscent of the Megaviridae.
This suggests that pandoravirus-like particles may correspond to an unexplored diversity of unconventional DNA virus families. The Megaviridae exhibit pseudo-icosahedral virions up to 0.7 μm in diameter and adenine–thymine (AT)-rich genomes of up to 1.25 Mb encoding a thousand proteins.
Except for few studies targeting the Influenza virus and the Variola virus over historical timespans (14, 15), the possibility that DNA viruses might remain infectious over a much longer time scale has not yet been investigated.On one hand, Megaviridae possess large pseudoicosahedral capsids enclosing a megabase-sized adenine–thymine-rich genome, and on the other, the recently discovered Pandoraviruses exhibit micron-sized amphora-shaped particles and guanine–cytosine-rich genomes of up to 2.8 Mb.While initiating a survey of the Siberian permafrost, we isolated a third type of giant virus combining the Pandoravirus morphology with a gene content more similar to that of icosahedral DNA viruses.Further sampling of various environments and geographical locations led to the isolation of Mimivirus variants (3, 4) and more distant relatives, two of which have been fully sequenced: Moumouvirus (5) and (6).All of these viruses share the same distinctive particle structure: a unique external fiber layer enclosing a pseudoicosahedral protein capsid of about 0.5 µm in diameter, itself containing lipid membranes surrounding an electron-dense nucleocapsid.