After Mimivirus, Mamavirus and the virophage, the group of giant viruses now has a new member called Marseillevirus.

Discovered in an amoeba by the team led by Didier Raoult at the Unité de Recherche sur les Maladies Infectieuses et Tropicales Emergentes research group (CNRS/Université Aix-Marseille 2), a description of this new virus was published online in the Proceedings of the National Academy of Sciences (PNAS).

Amoebae are single-cell, eukaryote (possessing a nucleus) living organisms, some of which are human or animal parasites and may cause a variety of pathologies. Most amoebae live in water, damp soils or mosses.

They are mobile and capable of ingesting a wide variety of different organisms (for example, viruses or bacteria with extraordinarily broadly ranging sizes and lifestyles). Thus amoebae provide a site for numerous exchanges of genetic material arising from the many organisms that "colonize" them.

The team led by Didier Raoult at URMITE (CNRS/Université Aix-Marseille 2)1 has recently discovered, in an amoeba, a member of a new family of giant viruses, which it has called the Marseillevirus, smaller than Mimivirus, which is the largest giant virus known at present.

With a chimeric genome (containing both DNA and RNA) of 368,000 base pairs, Marseillevirus is indeed the fifth largest viral genome to be sequenced. It has an icosahedral shape and a diameter of about 250 nanometers (or 250 millionths of a millimeter).

In addition, the researchers discovered that it contained genes from markedly differing sources, i.e. of bacterial, viral or eukaryote origin, or arising from Archae.2 The genome of Marseillevirus, a mosaic of genes from very different organisms, thus demonstrates the exchange of genes between the organisms that "colonize" amoebae.

"Marseillevirus at different stages of its formation in an amoeba. (Credit: Copyright Raoult / URMITE)"

Source: CNRS