Largest viral genome yet carries 2,300 genes that are new to biology

Meet the Pandoravirus, which carries more DNA than some bacteria.
Chantal Abergel and Jean-Michele Claverie

Up until recently, viruses seemed to be simple things. Too tiny to be seen except with an electron microscope, they had compact, efficient genomes that omitted just about any gene that could be supplied by their host cells. That view started to change with the discovery of giant viruses with genomes over a million base-pairs long. These viruses carry over a thousand genes, many of which would be present in their hosts’ genomes, and they can even be victims of smaller viruses.

But all of these giant viruses seemed to be related, and the viruses had a common life cycle once they infected the organisms that they preyed upon, so it was possible to view the Megaviruses as an oddball exception.

With a publication in today’s issue of Science, it appears to be time to leave that comforting thought behind. In the paper, the authors describe a virus with a genome roughly twice the size of the biggest Megavirus and a viral particle that’s so big it’s visible with a standard light microscope. The new virus appears completely unrelated to the Megaviruses, getting its own branch on the family tree. And it has a completely different life cycle than the Megaviruses, taking over its host cell’s nucleus in order to replicate.

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