Are Viruses Alive?

Are viruses alive? Research has uncovered a CRISPR-like immune system in a mimivirus. Mimiviruses are giant viruses that can be seen with a light microscope2. They usually infect amoebae and have very large genomes that encode proteins and enzymes used to make more copies of themselves. There has been some debate among the scientific community about whether viruses are alive or not. These newly discovered giant viruses add another argument for the “alive” category3.

Russel Chun for Quanta Magazine
Since mimiviruses encode their own replication machinery, and are so large, some scientists consider them to be more like prokaryotes than viruses. Like bacteria, they are also infected by other viruses called virophages4. This likely resulted in the development of a defense system that resembles the immune system in higher life forms. Prokaryotes encode a system called CRISPR (clustered regularly-interspaced short palindromic repeats), which is segments of DNA that contain repetitive sequences followed by short sequences of spacer DNA5. This enables prokaryotes to resist infection by phages and is a form of acquired immunity. CRISPRs act in a way similar to RNAi in eukaryotes – that is, they recognize and cut out exogenous genetic elements that are not part of the regular genome of the prokaryote. When foreign DNA enters the cell, if its sequence matches a sequence in the CRISPR library, Cas enzymes are called to action to unwind the foreign DNA and cut it up in order to stop the infection.
Scientists found CRISPR-like stretches of DNA in the genomes of certain strains of mimivirus1 that were resistant to infection by a virophage called Zamilon6. These Zamilon-resistant mimiviruses had genes that encode enzymes that, like Cas, can unwind and chop up DNA. When the scientists deleted or blocked these enzymes, it resulted in those mimiviruses getting infected by the Zamilon virophage. They named this system MIMIVIRE, and proposed that, since they have an immune-like system, mimiviruses should make up a fourth domain of life.
However, the mechanism of action of MIMIVIRE is not yet known. Other viruses have CRISPR-like components, but they have not been confirmed to function as defense systems7. It will be interesting to learn if these systems are indeed functional, and how they confer resistance to infection. Will they have similar applications as CRISPR, which is used for gene editing? I’m excited to see where this research leads. If you have any ideas about potential uses for MIMIVIRE, write us at tech@biolegend.com.
References:
  1. MIMIVIRE is a defence system in mimivirus that confers resistance to virophage
  2. Mimivirus inaugurated in the 21st century the beginning of a reclassification of viruses
  3. How the virophage compels the need to readdress the classification of microbes
  4. The virophage as a unique parasite of the giant mimivirus.
  5. CRISPR-mediated phage resistance and the ghost of coevolution past.
  6. Zamilon, a novel virophage with Mimiviridae host specificity
  7. Self-synthesizing transposons: unexpected key players in the evolution of viruses and defense systems

Contributed by Rea Dabelic, PhD.
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