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A Parasite’s Parasites

French scientists identify a new giant virus, which carries the genome of a smaller virus and a new breed of mobile DNA.

By | October 15, 2012

Desnues et al, 2012.In the contact lens fluid of a French woman with inflamed eyes, scientists have discovered several new parasites, nested inside one another like Russian dolls.

Bernard La Scola and Christelle Desnues from the CNRS in France found that the fluid was contaminated with an amoeba infected by a new giant virus, which they called Lentille virus. Inside that, they found a virophage—a virus that can only replicate in cells infected by another virus—which they called Sputnik 2. Deeper still, they found tiny chunks of parasitic DNA, which they called transpovirons, that can hop around Lentille virus genomes and stow away inside Sputnik 2. They published their results today (October 15) in the Proceedings of the National Academy of Sciences.

Lentille belongs to a group of giant viruses called Mimiviridae. First discovered in 2003, new members have been rapidly added to the family tree ever since. “This story again reveals another aspect of the remarkable complexity and diversity of giant viruses,” said Curtis Suttle, a virologist from the University of British Columbia who was not involved in the study.

Sputnik 2 is the fourth known virophage. La Scola and Desnues found the first—Sputnik—in 2008, under similar circumstances. It infected a giant virus called mamavirus, which infected an amoeba found in dirty water from a Parisian cooling tower. Mamavirus copies itself by creating large viral factories inside the amoeba, and Sputnik replicates by hijacking these factories at the giant virus’s expense.

Two more virophages were found in giant viruses last year—Mavirus, discovered by Suttle, and OLV, found in an Antarctic Lake by Ricardo Cavicchioli from the University of New South Wales.

Of the virophages found so far, however, Sputnik 2 holds a important distinction: it can insert its DNA into its host’s genome, just as other viruses such as HIV and herpes, insert their DNA into the genomes of infected animals.

Jean-Michel Claverie, also at the CNRS but not involved in this study, says that this discovery could explain why different giant viruses often carry similar genes. By fusing with the genomes of their hosts and hopping out again, virophages could act as “vehicles for gene transfer between these giant viruses,” he said.

The team also looked “in the trash,” said team leader Didier Raoult, examining DNA fragments found inside the giant viruses did not seem to be a part of the Lentille virus or Sputnik 2 genomes. In doing so, they found an independent stretch of DNA inside Lentille virus that outnumbers the giant virus’s own DNA by as much as 14 times and can insert itself into the virus’s genome or stay outside of it. Mobile pieces of DNA have been found in giant viruses before, but Raoult saw this as a new type, one with similarities to the transposons that jump in and out of the genomes of living cells. The researchers dubbed the new mobile element a transpoviron.

The transpovirons seem to rely on the giant virus to replicate, and while the details are unclear, these sequences are very good at reproducing. When Lentille virus first infects the amoeba, the transpovirons are “produced like mad” according to Raoult, at an earlier stage and to a greater extent than the genes of either the virophage or virus. They can also stow away inside Sputnik 2, and Raoult thinks that they may ride from one giant virus to another aboard virophages.

The transpovirons seem to be a mish-mash of DNA from many sources. They contain between six to eight genes, including some that look to have come from giant viruses, others that are similar to virophage genes, and at least one that may have come from bacteria.

The same is true for virophages themselves. Sputnik’s tiny genome contains genes that look like they came from giant viruses, bacteria, or eukaryotic cells. Mavirus has genes that look like jumping genes called “Maverick transposons,” which are also found in eukaryotic cells, including those in humans. These eukaryotic Maverick sequences may evolved from virophages. Since virophages hamper the replication of giant viruses, Suttle suspects that early cells might have protected themselves from such viruses by incorporating virophages into their own genomes, effectively domesticating them.

Raoult suspects that many more virophages and transpovirons await discovery. “Very few people work on giant viruses, and [transpovirons] may have been neglected as they are unexpected,” he said. His team has already found more such elements in the genomes of three other giant viruses.

Suttle agrees. “The natural viral world encompasses the greatest genetic and biological diversity on Earth,” he said. “Its continued exploration will undoubtedly unlock many more secrets that fundamentally change our understanding of the evolution and diversity of life on our planet.”

C. Desnues et al., “Provirophages and transpovirons as the diverse mobilome of giant viruses,” Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1208835109, 2012.

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Comments

Avatar of: Curculio

Curculio

Posts: 49

October 16, 2012

The term virophage will not stand the test of time. Phage is derived from the Greek -phagein- to eat, a concept easily seen in bacteriophages when they literally explode a cell.  This aspect doesn't work for these viruses as they have more in common with inquiline social parasites than they do with bacteriophages.  Even then the analogy is rough so I would suggest a new class of parasitism for them.  They hinder reproduction of their vector host, which limits their own ability to move.  They do so by promoting their own growth at the expense of both the primary cell host and the vector host.  I would call them Virovenivici, which is a virus that came and conquered.  This term is alliterative and emphasizes both essential aspects of its life history with the mimivirus and its cell host: travelling (veni) and dominance over both cell and vector host (vici). Yes, this would be a Greek-Latin hybrid term but remember, we live in the age of multiculturalism.

Avatar of: Horacioh

Horacioh

Posts: 3

October 17, 2012

Very nice and so interesting.

Avatar of: cyberdrcarr

cyberdrcarr

Posts: 1

October 17, 2012

This is an exciting discovery.  It will lead to new models on the origin and evolution of life.  It provides support to the idea the  NCLD viruses evolved from archaea like cells that became parasites.  Likewise the virophages evolved from earlier  parasitic cells and their mobile genetic elements (transvirons )may represent the earlies forms of replicative genetic elements that arose before the evolution of these cells.  These developments will give us a better understanding of how cells evolved.  My synthesis of some of these discoveries is discused here http://www.telemedical.com/prokaryoticevolution.htm .

Avatar of: Horacioh

Horacioh

Posts: 3

October 17, 2012

Very interesting and nice article.

Avatar of: vinnisha

vinnisha

Posts: 9

October 24, 2012

This is a very interesting and captivating article that is so helpful to so many people out there. I guess viruses may sometimes aid people to a better life, not only on a bad side, but trying to be more optimist on the postive side of the viruses.

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