MIMIVIRE: evidence for a new form of life?
By Dana Nieh '19
The mimivirus has been stirring up controversy within the scientific community due to its potential categorization as the unrecognized fourth branch of the tree of life--in addition to bacteria, archaea, and eukaryota--as has been argued for years by co-authors Didier Raoult and Bernard La Scola of the study published in Nature. Unlike a typical virus, a mimivirus is four times larger and is similar to prokaryotes in its ability to make amino acids, nucleotides, and complex proteins without containing any nuclei, thereby supporting this potential classification.
The study, published in Nature Magazine on leap day, was conducted by microbiologists at Aix-Marseille University and has revealed a CRISPR-like defense system that combats virophage infection in mimiviruses. This viral defense system, coined “mimivirus resistance virophage element (MIMIVIRE)” by the study’s authors, was discovered by identifying parts of DNA from the Zamilon virophage within the genomes of sixty strains of mimiviruses. The strains containing chunks of Zamilon DNA within their own genetic code were able to identify and attack invading Zamilon. When the researchers blocked the genes encoding the enzymes t hat degrade and unwind the viral DNA, the mimivirus was left susceptible to Zamilon attack. This experiment draws similarities between mechanisms of MIMIVIRE in viral strains and CRISPR in bacterial strains, thereby confirming the team’s hypothesis addressing its significance in conferring virophage resistance.
Furthermore, lead researcher Raoult purports that this defense system is an ancient adaptation, an example of convergent evolution with bacteria’s CRISPR system, that supports the authors’ claims that the mimivirus constitutes an entirely new domain of life. Viral researcher Chantal Abergel explains, “this [discovery] blurs the frontier between viruses and cells and asks for reconsideration of what should be considered as alive.” However, many scientists argue that because the mimiviruses have no autonomy, do not metabolize, and depend on living organisms to reproduce, they instead exist “at the border between chemistry and life.” Virologist Marc H. V. van Regenmortel of the University of Strasbourg in France supported this notion by claiming that their dependence on host cells force viruses to lead “a kind of borrowed life.”
Regardless of the mimivirus’ domain classification, microbiologist Francisco Mojica of the University of Alicante in Spain, who is notable for his identification of prokaryotic CRISPR sequences in the 1990s, has emphasized the importance of identifying the specific mechanisms involved in MIMIVIRE immunity, which he suspects will differ significantly from those in CRISPR immunity. The discovery holds immense potential for novel applications in genome editing technology and other fields.
Sources:
http://www.nature.com/news/crispr-like-immune-system-discovered-in-giant-virus-1.19462
http://www.nature.com/nature/journal/vaop/ncurrent/full/nature17146.html#close
http://gizmodo.com/giant-viruses-feature-their-own-built-in-antivirus-soft-1762030514
http://phys.org/news/2016-03-giant-viruses-immune-similar-bacteria.html
http://www.nature.com/nature/journal/vaop/ncurrent/fig_tab/nature17146_SF2.html