New Giant Viruses Further Blur the Definition of Life

Jordana Cepelewicz in Quanta:

ScreenHunter_3035 Apr. 08 20.33For decades, descriptions of viruses have straddled life and nonlife, a divide that usually isn’t difficult to navigate. Their hallmark characteristics, namely their small size, tiny genomes and parasitic dependence on cellular hosts for replication, set them apart from all other living things despite their animation. But that story has gotten far more puzzling — particularly since the discovery of the first giant virus in 2003, which was so large that researchers initially thought it was a bacterium.

Several families of giant viruses are now known, and some of those giants have more than 1,000 genes; one has a whopping 2,500. (By comparison, some small viruses have only four genes.) Among those genes are ones involved in translation, the synthesis of proteins — a finding that came as a shock. “It appears that giant viruses are as complex as living organisms,” said Chantal Abergel, an evolutionary biologist at Aix-Marseille University in France.

That conclusion was reinforced last week when scientists reported in Nature Communications that they had found two new giant amoeba-infecting viruses in Brazil, which they named tupanviruses (after Tupã, a thunder god of the regional people). Tupanviruses are striking, and not just because they possess long tails: They have the most complete set of translation-related genes seen to date, including those for all 20 of the enzymes that determine the specificity of the genetic code. The only components they are missing are full-length ribosomal genes. Whether all those elements actually function still needs to be tested.

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