Scientific Method / Science & Exploration

Giant virus found in tiny predator

A giant virus has its genome sequenced, which shows that it has stolen genes …

A few years back, a giant virus made news because it blurred the boundaries between what we consider living cells and viruses. The Mimivirus, which infected a freshwater amoeba, had a genome that contained over a million base pairs of DNA, and carried a set of genes that were previously only found in living cells. In a PNAS paper that will be released later this week, researchers describe the genome of an oceanic mimivirus cousin that has the second largest viral genome ever seen. It was apparently discovered off the coast of Texas; you can insert your own jokes here.

The virus was found in a single-celled host that preys on the bacteria and plankton at the base of the food chain. The new find, Cafeteria roenbergensis virus (CroV), has a genome that's over 700,000 base pairs long (700 kilobases, or kb). It's linear, and the ends are filled with repetitive DNA, which the authors speculate acts a bit like telomeres do in human cells, protecting the important DNA. The gene-containing region is about 630kb, and encodes about 550 proteins, along with a handful of transfer RNAs (part of the protein production machinery).

The virus also carries some molecular parasites called inteins. These pieces of DNA insert into the middle of genes and encode a sequence that ends up in the middle of its host's protein once the gene is translated. This doesn't actually disrupt the host's protein, though—amazingly, the intein protein sequence can cut itself out of the surrounding host protein and link its two ends together as if the intein were never there. The excised intein protein then helps ensure that the DNA sequence that made it gets inherited and spreads.

About half of the virus' genes are similar to those of something from either other giant viruses or living cells, with pieces from all three domains of life (eukaryotes, bacteria, and archaea). But the majority of genes have no known function, so it's hard to know what to make of them. Some of the ones that are present, however, are pretty sophisticated. The virus has its own DNA repair system, and can hijack a system its host uses to destroy unwanted proteins; it uses this to get rid of the host's defense proteins. It also seems to have picked up 38kb from a bacteria (potentially, one of the meals of its host) that encodes for a pathway that attaches sugars to proteins.

When mimivirus was first identified, some of its discoverers looked at its large size and distinct genome and decided that viruses like that may have participated in the origin of life. CroV, the new virus, is actually the mimivirus' closest known relative, but the two viruses only share about a third of their genes, and many of the critical ones have come from completely different species. Thus, the viruses seem to have been built from a small, ancient core that has undergone a lot of horizontal gene transfers that provided them with genes from living cells.

This doesn't rule out a possible ancient existence for this group of viruses, but it does suggest that the ancient versions probably looked very little like the modern ones. Which probably means that the argument over their role in the origin of life is going to be very difficult to settle, and may go on for years.

PNAS, 2010. DOI: 10.1073/pnas.1007615107  (About DOIs).

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