Scientists have discovered a new family of giant viruses -- created within amoebae, they report in this week's issue of the Proceedings of the National Academy of Sciences.

Structural analysis of giant virus Image: Xiao C, Kuznetsov YG, Sun S, Hafenstein SL, Kostyuchenko VA, et al. (2009)

The new virus type is uniquely comprised of genes from a variety of origins, including bacteria, eukaryotes and viruses.

Amoebae are not an uncommon source of viruses, since their insides are melting pots of viruses and other organisms, allowing viruses to grow into so-called "giants" by adopting genes from other organisms within the amoebae.

"Researchers have recognized the potential of amoebae as a source of new viruses for a while," said Patrick Forterre, a microbiologist at the Pasteur Institute in Paris, France, who was not involved with the study. "But [this paper] is the first real confirmation that [other giant viruses found in amoebae are] not an exception... It also means there might be a huge number of diverse giant viruses in amoebae that haven't been discovered yet."

Giant viruses were first discovered in the late 1990s, and are characterized by their large particle sizes, typically bigger than 200 nanometers, and genetic complexity. (Medium-sized viruses such as adenovirus and HIV measure closer to 100-200 nm.) This newest giant virus was discovered by Didier Raoult and colleagues the Universite de la Mediterranee in Marseille, France. The scientists isolated the new virus, named Marseillevirus, from Acanthamoeba polyphaga.

Phylogenetic analysis of the Marseillevirus revealed several genetic similarities to other giant viruses. It also carries genes obtained from eukaryotic hosts and their parasites or symbionts. "Other viruses are commonly alone in their host cell," says Raoult. But because amoebae feed on relatively large particles, over 500nm, they are one of the few organisms that can take in and host a giant virus. And since amoebae are not picky eaters, they typically contain a mixture of organisms.

The giant virus adopts genes from the other organisms, including eukaryotes, bacteria and other viruses, that improve function. In the case of the Marseillevirus, genes with defense or repair functions are thought to have come from bacteria; those with metabolic functions likely have eukaryotic and bacteria origins; and those for signal transduction probably stem from eukaryotes, the scientists report.

The team's findings led them to conclude that amoebae are "melting pots" for viruses, said Raoult, enabling viruses to create complex gene repertoires with varied genetic origins.

It is unclear whether this new giant virus is pathogenic, but may turn out to be so, said Raoult, because of the virus's "repertoire of genes and the capability to resist intracellular killing."


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[6th August 2008]