Didier Raoult of Aix-Marseille University in France and his colleagues discovered a new kind of virus lurking inside single-celled protozoans back in 2003. Like other viruses, it couldn’t grow on its own, lacking the biochemical machinery to build proteins and genes. Instead, it had to infect host cells and use their material to produce new viruses.
But this new virus was enormous, measuring hundreds of times bigger than any previously known virus. What’s more, it was far more complex. Typical viruses may have just a few genes. The new virus had over 900 — more than many species of bacteria.
Since then, Raoult and his colleagues have found over 150 different kinds of giant viruses all over the world, in oceans, mountains, and the bodies of animals (including our own). One kind of giant virus contains over 2,500 genes.
Exactly what giant viruses do with all those genes has remained mostly a mystery.
But on Monday, Raoult and his colleagues reported in Nature that some of those genes provide giant viruses with something never observed before in a virus: They have an immune system, one that works a lot like the CRISPR system in bacteria that scientists have co-opted as a powerful gene editing tool.
>the potential for such a system to be harnessed for genetic control is intriguing
>Raoult and his colleagues first discovered that giant viruses get infected with viruses of their own back in 2008.
These so-called virophages slip inside the giant viruses and hack their biochemistry, much as the giant viruses do to their own protozoan hosts.
>One of these virophages, called Zamilon, infects a type of giant virus known as a mimivirus. But when Raoult and his colleagues unleashed Zamilon on closely related strains of mimiviruses, they were surprised to find that it couldn’t infect them.
>It appeared as if the giant viruses could defend themselves against their enemies.
Raoult and his colleagues wondered if giant viruses were using a CRISPR-like defense system against Zamilon. To their surprise, they found that resistant giant viruses carried small pieces of the virophage’s DNA in their own genomes. When they searched the DNA that surrounded the Zamilon sequences, they found a gene that unwinds DNA, and another that slices it.
>The scientists hypothesized that giant viruses used these two genes to chop up Zamilon DNA. To test that idea, they silenced each of the genes. Now, the giant viruses became vulnerable, and Zamilon was able to infect them.
>Raoult and his colleagues have dubbed this stretch of giant virus DNA MIMIVIRE, short for “mimivirus virophage-resistance element.” They propose that it serves as an immune system, although they have yet to determine how the giant virus recognizes virophages and directs enzymes to attack it.
“What we know is that it’s critical,” said Raoult. “If you silence the genes, it doesn’t work anymore.”
Raoult said that like CRISPR, MIMIVIRE might be worth investigating as another potential gene editing tool: “It is different, so it may have different applications.”
Even if that search bears no fruit, Raoult thinks that MIMIVIRE is important for what it says about the evolution of giant viruses.
https://archive.is/9lev1
MIMIVIRE