The mimivirus is so enormous it has its own kind of CRISPR-like immune system to defend against the smaller viruses that attack it. A team in France has confirmed how it works by transferring the entire system to a bacterium and tweaking it to destroy a different target.
While CRISPR has become famous as a tool geneticists can use for editing genomes, it evolved in bacteria as a way of defending against viruses. The bacteria 鈥渃annibalise鈥� bits of DNA from the viruses that attack them and add this DNA to their own genomes.
The CRISPR system allows the bacteria to recognise and destroy any matching viral DNA the next time they get attacked. In other words, CRISPR acts like an adaptive immune system.
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In 2016, Didier Raoult of Aix-Marseille University in France sparked controversy when he claimed that that works in a similar way to CRISPR, dubbed MIMIVIRE.
Giant viruses are strange entities first identified in 2003. Like all viruses, they cannot multiply independently but rely on hijacking other organisms. But while most viruses are little more than protein shells containing DNA or RNA, giant viruses have lots of active cellular machinery inside them, such as protein-making factories.
They are so complex they are even plagued by their own viruses, or virophages. The first virophage was discovered by Raoult in 2008.
His team also discovered that one strain of mimivirus is immune to a virophage called zamilon. This mimivirus strain has bits of zamilon DNA integrated into its genome, which it uses to recognise and chew up the DNA of invading zamilon virophages.
But critics .
So Raoult has now transferred the MIMIVIRE system to the E. coli bacterium. Crucially, the team swapped the zamilon sequences for bits of a bacterial gene. When they activated MIMIVIRE, that bacterial gene got chewed up.
鈥淲e changed the target,鈥� says Raoult.
Lamarckian
This shows that the system is not specific to zamilon, and that a mimivirus could almost instantly by cannibalising its DNA, he says. His team is now searching for mimivirus strains that have done exactly this.
鈥淚t鈥檚 very Lamarckian. It鈥檚 not Darwinian,鈥� says Raoult, referencing an old debate on the way evolution occurs. Lamarck thought species evolved on the basis of experiences individuals gained during life 鈥� with, for instance, giraffes gaining longer necks by stretching to reach higher leaves.
Raoult thinks many other microbes harbour as-yet-undiscovered defence mechanisms based on cannibalising the DNA of attackers and using it against them. 鈥淚 think it is very very common and we will discover more,鈥� he says.
It might be possible to adapt MIMIVIRE for editing genomes, as has been done with several of the thousands of variants of the CRISPR system found in bacteria and archaea. But Raoult has no plans to try this.
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