Pithovirus sibericum

This student page has not been curated.

Classification

Viruses; dsDNA viruses, no RNA stage; unclassified dsDNA viruses.(Source: [1])

Species

NCBI: Taxonomy

Pithovirus Sibericum

Description and Significance

Pithovirus sibericum is a giant DNA virus of a previously unclassified family isolated from a layer of permafrost in the Kolyma Lowland region of Siberia in 2013. The sedimentary layer from which the Pithovirus was isolated dates back 30,000 years or more. Pithovirus infects the amoebae; human and animal pathogenicity has been ruled out thus far. Pithovirus is notable due to its unprecedented size compared to known viruses, its unusually small genome relative to its size, and its lack of phylogenetic relationship to any known virus family.

Genome Structure

Pithovirus sibericum’s genome is unexpectedly different from that of other giant DNA viruses with morphological similarity. Pithovirus has an AT-rich genome with a mere 610,033 bp and encodes only 467 proteins; morphologically similar viruses feature GC-rich genomes with up to 1.26 Mb and 2,500 protein-coding genes. Pithovirus’s genome overall structure is currently uncertain, but appears to be either linear with terminal redundancy or circular.

Examination of Pithovirus’s protein-coding genes shows that 67% have no homologs in any known sequence. Of the 33% that are homologous to known proteins, they are equally comparable to viruses, bacteria, and eukaryotic organisms, indicating that Pithovirus has no particularly close phylogenetic relationship to any known sequenced organism. 11% of the total genome corresponds to proteins sequenced from viruses. Phylogenetic analysis places Pithovirus in a clade of icosahedral large DNA virus families and most closely related to Marseilleviridiae.

   [[File:2014	Pithovirus sibericum genome.jpg|frame|Right|Size|alt=Alt|Pithovirus Genome]]

Cell Structure, Metabolism and Life Cycle

The Pithovirus is an oblong rod with a “cork” at its apex that is lost from the virus during replication. The virus features an external envelope and an internal lipid membrane. Within the internal membrane a tubular structure of unknown function has been observed.

Pithovirus infects amoeboid cells by expelling its cork and fusing its internal lipid membrane to the host cell membrane, allowing the virus to establish a virion assembly within the host cell cytoplasm. Once virion assembly is complete, the virus is released via host cell lysis.

Ecology and Pathogenesis

Pithovirus sibericum is a pathogen of amoeba and proved virulent after 30,000 years frozen in permafrost. This discovery of a millenia-old pathogen has important implications on the effects of human activity to soils and sediments. Furthermore, climate change has the potential to alter icy ecosystems and release previously unknown microorganisms. The possibility that ancient microbes are revived upon release from soil highlights the importance of attending to potential pathogenicity.

References

[2] [3]

Author

Page authored by Jennifer Gliessman, student of Prof. Jay Lennon at IndianaUniversity.