After the Megaviridae (represented by Mimivirus, discovered in 2003), Pandoraviridae (discovered in 2013) and Pithovirus (described in 2014) families, a fourth family of giant viruses infecting amoebae of the Acanthamoeba genus was discovered by the research team responsible for isolating Pithovirus. By persevering in the study of the frozen soil sample from the extreme north-east of Siberia in which Pithovirus had been found, the scientists were able to isolate, amplify and then characterize the new virus, Mollivirus sibericum. This is the first time that all the analytical techniques applicable to living beings - genomics, transcriptomics, proteomics and metagenomics - have been used simultaneously to characterize a virus.
The virus takes the form of a roughly spherical particle, approximately 0.6 μm long, containing a genome of approximately 650,000 base pairs coding for more than 500 proteins. Most of these proteins bear no resemblance to those of its Siberian predecessor, Pithovirus sibericum. Furthermore, unlike Pithovirus, which only requires the cytoplasmic resources of its cellular host to multiply, Mollivirus sibericum uses the cell nucleus to replicate[1] in the amoeba, which makes it as host-dependent as most "small" viruses. This strategy, and other specific traits, such as a deficiency in certain key enzymes that allow synthesis of its DNA building blocks, mean that Mollivirus sibericum is more similar to the common viral types, including human pathogens such as Adenovirus, Papillomavirus, or Herpesvirus. Pithovirus, on the other hand, replicates in the cytoplasm in the same way as Poxvirus, a family that counts the now officially eradicated smallpox virus. In terms of its shape, mode of replication and metabolism, Mollivirus sibericum thus represents a new type of virus never previously observed and distinct from the three giant virus families discovered to date.
This discovery, which suggests that giant viruses are not so rare and are highly diversified, also proves that the ability of viruses to survive in the permafrost for very long periods is not restricted to a particular viral type, but probably covers viral families with varied — and hence potentially pathogenic — replication strategies. The results of the metagenomic analysis of this permafrost sample, which revealed very low concentrations of Mollivirus (around a few parts per million), have important public health implications today. In the presence of a susceptible host, a few particles that are still infectious could indeed be sufficient to cause the resurgence of potentially pathogenic viruses in Arctic regions that are increasingly coveted for their mining and oil resources, and whose accessibility and industrial exploitation have been facilitated by climate change.
In order to determine whether other giant viruses are still hidden in the permafrost, the scientists are now studying[2] even more ancient layers of the Siberian soil, working in a region that should enable them to go back a million years.
[1] Viral replication is the series of processes that occur in a cell infected by a virus, resulting in the production of new units of this virus (or virions).
[2] With the support of national France-Génomique ProFi infrastructures (Investissements d'avenir).