Giant Virus Has Its Own Antiviral Immune System

Mimiviruses are as big as some bacteria. vanou/Flickr (CC BY-NC-ND 2.0)

It’s a dog-eat-dog world out there, and even viruses aren’t safe from attack by their own kind. But how does something so small, so simple, protect itself from invaders? It turns out that some truly unique viruses, called mimiviruses, actually have a kind of antivirus system that lets them store information on attackers, prepping themselves for future rounds of assault.

Intriguingly, it seems to work like the natural defense mechanism found in bacteria, called CRISPR, which scientists borrowed to create a highly efficient and precise gene-editing tool. The presence of such an “immune system” only adds to the debate as to whether viruses, or at least some, should be classed as living organisms, something that the lead author of this present study is arguing the case for, according to Nature News.

Giants of the virus world, mimiviruses make you think that everything they taught you about viruses in school is a lie. Measuring more than 0.7 micrometers across, they’re visible under a microscope and rival the size of a typical parasitic bacterium. They also contain more than 1,000 genes, a staggering number when compared to a virus like HIV, which has just nine. But what is perhaps most fascinating about these massive viruses is that they possess the equipment to make their own proteins, a process that viruses typically need to hijack a cellular host for, hence their definition as “obligate intracellular parasites.”

It seems the unique nature of these viruses does not end there, though. Some viruses fatally infect bacteria, and these are called bacteriophages. Similarly, those that destructively target viruses are called virophages, and one in particular has been found to go for mimiviruses. Interestingly, this virophage, called Zamilon, is only capable of infecting certain mimivirus strains, leading researchers to ponder whether some had developed a kind of immunity to them, perhaps similar to the one employed by bacteria – CRISPR.

This system works by the bacteria collecting an archive of bits of genetic material stolen from invaders during attacks, which can be used to quickly identify future threats from the same assailant. If confronted again, the bacterium chops up the attacker’s genome and escapes a potential fatality. So if viruses have evolved similar tactics, one would expect to find strands of Zamilon stashed away inside the mimivirus, which is precisely what the team from Aix-Marseille University in France searched for.

Described in Nature, that was indeed the case, with the researchers discovering its genome laced with repeat sequences of Zamilon DNA. And to demonstrate their defensive purpose, the researchers switched off these sequences and presented the resulting mimiviruses with Zamilon, which they were now susceptible to. But this wasn’t all they found: Adjacent to these sequences were genes responsible for the degradation of foreign DNA, again reminiscent of CRISPR. The team has coined this system MIMIVIRE.

“Our findings illustrate that giant viruses have undergone genetic evolution that is similar to other microbes,” the researchers conclude. Whether the presence of this system is enough to win the living vs. non-living argument remains to be seen, but it’s an interesting discovery nonetheless. 

Image in text: vanou/Flickr (CC BY-NC-ND 2.0)

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