The saga of giant Acanthamoeba-infecting viruses started in 2003 with the discovery of Mimivirus. Two additional types of giant viruses have been discovered since: the Pandoraviruses (Pandoravirus salinus and P. dulcis) and Pithovirus sibericum, the latter one revived from 30,000-year-old permafrost in Siberia. A team of French scientists now describes a fourth type of giant virus isolated from the same permafrost sample.
Mollivirus sibericum particle at a late assembly stage. The particle is crowned with several fuzzy rings, and different tegument layers are visible. Image credit: Matthieu Legendre et al.
Microscopic, genomic, transcriptomic, proteomic and metagenomic technologies have allowed the team, led by Dr Chantal Abergel and Dr Jean-Michel Claverie of the Aix-Marseille University, to draw a detailed portrait of this new virus, named Mollivirus sibericum.
According to the scientists, Mollivirus sibericum takes the form of a roughly spherical particle, around 600 nm long.
“Mollivirus sibericum was initially spotted using light microscopy as rounded particles multiplying in a culture of Acanthamoeba castellanii inoculated with a sample of Siberian permafrost from the Kolyma lowland region,” they wrote in a paper published in the Proceedings of the National Academy of Sciences.
“After amplification, the particles were analyzed by transmission electron microscopy and scanning electron microscopy. Mollivirus sibericum’s roughly spherical particles are 500-600 nm in diameter and appear surrounded by a hairy tegument.”
Mollivirus sibericum has a relatively large genome of 650,000 base pairs encoding for only 523 proteins. Most of these proteins bear no resemblance to those of its Siberian relative, Pithovirus sibericum.
Furthermore, unlike Pithovirus sibericum, which only requires the cytoplasmic resources of its cellular host to multiply, the new virus uses the cell nucleus to replicate 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 sibericum, on the other hand, replicates in the cytoplasm in the same way as Poxvirus.
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 other families of giant viruses.
“Until Mollivirus sibericum relatives are isolated in contemporary environments, we cannot rule out that the permafrost was the only reservoir left for this viral family,” the researchers wrote in the paper.
The discovery 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.
“Our finding that two different viruses infecting the same host could be revived from a single permafrost sample, definitely suggests that prehistory ‘live’ viruses are not a rare occurrence.”
“Furthermore, the roughly equal representation of the two viruses in the metagenomics data suggests that there is no difference in the survival capacity of particles of either cytoplasmic (Pithovirus) or nucleus-dependent viruses (Mollivirus). Such modes of replication also correspond to the Poxvirus and Herpesvirus families, respectively.”
“Although no read sequences were close enough to detect known Poxvirus and Herpesvirus isolates in the metagenome of our permafrost sample, we cannot rule out that distant viruses of ancient Siberian human (or animal) populations could reemerge as arctic permafrost layers melt and/or are disrupted by industrial activities,” the scientists concluded.
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Matthieu Legendre et al. In-depth study of Mollivirus sibericum, a new 30,000-y-old giant virus infecting Acanthamoeba. PNAS, published online September 8, 2015; doi: 10.1073/pnas.1510795112
