The first gigantic mimiviruses were discovered in 2007 by a research group in France. They named the virus they found “Mimivirus,” originally thought to be a bacterium because of its size, due to its diameter (0.8 µm) and genome size (1.2Mb) “mimicking” the dimensions of a bacterium (Clinical Infectious Diseases
). Almost ten years later, the same group, led by Didier Raoult, PhD, has made another monumental discovery: a CRISPR-like immune system held by mimiviruses.
CRISPR system DNA sequences were discovered in bacteria by a Spanish microbiologist named Franciso Mojica in the 1990s. Just like humans, bacteria have to defend themselves against viral infections from microbes that specifically target bacteria, called bacteriophage. The CRISPR system works by keeping a record of short DNA sequences that correspond to sequences of the bacteriophage. When the viruses attack, the CRISPR systems identifies them as a threat and engages certain gene sequences coding for Cas enzymes that break down the bacteriophage DNA. After its discovery, the CRISPR system was “repurposed” as a gene editing technique serving a variety of purposes.
Now a similar defense system has been discovered in mimiviruses, whose genomes are sometimes larger than some bacterial genomes. They are also visible under a light microscope, and despite similarities to other viruses like smallpox, mimiviruses also have bacteria-like genes that make amino acids, DNA nucleotides, and complex proteins. Mimiviruses also require protection from bacteria and other viruses. Like bacteria, mimiviruses have a specific type of viruses that are “out to get them,” called virophage.
Raoult and his team studied 60 mimivirus strains and their genomes, looking for DNA sequences that correspond to a common virophage, called Zamilon. After finding evidence of such sequences, researchers also found genes encoding enzymes to degrade and unwind DNA, just like Cas enzymes do in the bacterial CRISPR system. Additionally, both systems encode the degrading enzymes adjacent to the DNA sequences that correspond to the the bacteriophage and virophage.
Raoult believes that this predator-prey relationship between mimiviruses and virophage prompted the evolution of the mimivirus CRISPR-like immune system. In his new study published in Nature
, blocking activity from parts of the mimivirus CRISPR-like system greatly increased mimivirus susceptibility to virophage infection.
“They are facing the same kind of challenge that prokaryotes have when they live in communities,” he said. Raoult also suspects that mimiviruses secrete antibiotic compounds in addition to their CRISPR-like immune system to prevent bacterial infections. This opens a whole new discussion into the possibility of developing new antibiotics from mimiviruses.
Raoult dubbed the CRISPR-like immune system of mimiviruses to be called “MIMIVIRE,” and he believes this distinctive discovery makes a claim for mimiviruses to have their own “branch on the tree of life.”