A new type of giant virus
called "Pithovirus" has been discovered in the frozen ground of
extreme north-eastern Siberia by researchers from the Information Génomique et
Structurale laboratory (CNRS/AMU), in association with teams from the Biologie
à Grande Echelle laboratory (CEA/INSERM/Université Joseph Fourier), Génoscope
(CEA/CNRS) and the Russian Academy of Sciences. Buried underground, this giant
virus, which is harmless to humans and animals, has survived being frozen for
more than 30,000 years. Although its size and amphora shape are reminiscent of
Pandoravirus, analysis of its genome and replication mechanism proves that
Pithovirus is very different. This work brings to three the number of distinct
families of giant viruses.
In the families Megaviridae
(represented in particular by Mimivirus, discovered in 2003) and Pandoraviridae,
researchers thought they had classified the diversity of giant viruses (the
only viruses visible under optical microscopy, since their diameter exceeds 0.5
microns). These viruses, which infect amoebae such as Acanthamoeba, contain a
very large number of genes compared to common viruses (like influenza or AIDS,
which only contain about ten genes). Their genome is about the same size or
even larger than that of many bacteria.
By studying a sample from the
frozen ground of extreme north-eastern Siberia, in the Chukotka autonomous
region, researchers were surprised to discover a new giant virus more than
30,000 years old (contemporaneous with the extinction of Neanderthal man),
which they have named Pithovirus sibericum. Because of its amphora shape, similar
to Pandoravirus, the scientists initially thought that this was a new member --
albeit certainly ancient -- of this family. Yet genome analysis on Pithovirus
showed that this is not the case: there is no genetic relationship between
Pithovirus and Pandoravirus. Though it is large for a virus, the Pithovirus
genome contains much fewer genes (about 500) than the Pandoravirus genome (up
to 2,500). Researchers also analyzed the protein composition (proteome) of the
Pithovirus particle (1..5 microns long and 0.5 microns wide) and found that out
of the hundreds of proteins that make it up, only one or two are common to the
Pandoravirus particle.
Another primordial difference
between the two viruses is how they replicate inside amoeba cells. While
Pandoravirus requires the participation of many functions in the amoeba cell
nucleus to replicate, the Pithovirus multiplication process mostly occurs in
the cytoplasm (outside the nucleus) of the infected cell, in a similar fashion
to the behavior of large DNA viruses, such as those of the Megaviridae family.
Paradoxically, in spite of having a smaller genome than Pandoravirus,
Pithovirus seems to be less reliant on the amoeba's cellular machinery to
propagate. The degree of autonomy from the host cell of giant viruses does not
therefore appear to correlate with the size of their genome -- itself not
related to the size of the particle that transports them.
In-depth analysis of
Pithovirus showed that it has almost nothing in common with the giant viruses
that have previously been characterized. This makes it the first member of a
new virus family, bringing to three the number of distinct families of giant
viruses known to date. This discovery, coming soon after that of Pandoravirus,
suggests that amphora-shaped viruses are perhaps as diverse as icosahedral
viruses, which are among the most widespread today. This shows how incomplete
our understanding of microscopic biodiversity is when it comes to exploring new
environments.
Finally, this study
demonstrates that viruses can survive in permafrost (the permanently frozen
layer of soil found in the Arctic regions) almost over geological time periods,
i.e. for more than 30,000 years (corresponding to the Late Pleistocene). These
findings have important implications in terms of public health risks related to
the exploitation of mining and energy resources in circumpolar regions, which
may arise as a result of global warming. The re-emergence of viruses considered
to be eradicated, such as smallpox, whose replication process is similar to
Pithovirus, is no longer the domain of science fiction. The probability of this
type of scenario needs to be estimated realistically. With the support of the
France-Génomique infrastructure, set up as part of the national Investments for
the Future program, the "Information Génomique et Structurale"
laboratory is already working on the issue via a metagenomic study of the
permafrost.
While there is a collective
fear for microorganisms for causing human diseases in particular, many of them
are actually beneficiel in the field of food, vehicle and antibiotic
production. Koyal Info Mag
prides itself in its wide coverage of scientific news, discoveries and
resources that caters to researchers, scientists, students, scholars,
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The above article is a repost from
ScienceDaily
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