Didier Raoult, a microbiologist at Aix-Marseille University in France, must be some sort of virus whisperer. He'd made various discoveries about the world of viruses over the years. In the latest study that he co-led with fellow microbiologist Bernard La Scola, for instance, they discovered that a type of giant virus called mimivirus has an immune system and can protect itself against other viruses. Raoult believes the study's findings are proof that mimiviruses, which typically grow inside amoeba, constitute a separate and fourth domain in the Woese system of classification. Under the Woese system, the three recognized domains of cellular life are Bacteria, Archaea (single-celled microorganisms) and Eukarya (cells with membranes and organelles).
Let's go back a few years to understand the whole picture here. Raoult and his colleagues discovered the existence of complex, gigantic viruses over a decade ago. They're big enough to be seen under an ordinary microscope, whereas typical viruses require the use of more powerful electron microscopes. Five years later, back in 2008, Raoult and the team he was working with found that those giant viruses can get sick when infected by other viruses now called "virophages."
When Raoult and La Scola tried to infect mimiviruses with a virophage known as Zamilon, they found that the big viruses were able to fight the infection off. Upon taking a closer look, the team determined that the mimiviruses employ an immune system similar to CRISPR. That's short for "clustered regularly-interspaced short palindromic repeats," which is the immune system found in a lot of bacteria and archaea.
The MIMIVIRE system (as Raoult chose to name it) can store genetic materials from the virophages that it comes into contact with. It then consults this collection whenever it senses the presence of an invader. If it's a virophage the system remembers, it sics enzymes upon the intruder to chop its DNA up into pieces.
While the researchers still have to figure out MIMIVIRE's mechanism, Raoult thinks its presence inside mimiviruses proves that these big organisms are ancient and deserving of their own domain. He also believes that just like CRISPR, it could be used in the lab to edit genes for research.