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A couple little "Sputnik" viruses seen as green blobs infect part of the bigger mamavirus, which is the big red blob.  (Source: La Scola, B. et al. Nature doi:10.1038/nature07218 (2008)

Big viruses may be an integral, and perhaps living, part of the ocean plankton ecosystem which impacts both the entire ocean and the global climate.   (Source: J. SCHMALTZ/NASA)
Looks like viruses can catch colds just like the rest of us

One of the fundamental questions in science is "what is life?", a question particularly pertinent of late with the search for signs of life on Mars and the advances in developing synthetic life in a lab.  Scientists have devised many complex answers to the question, but basically most scientists would agree that a "living" creature must be able to produce a variety of useful structural units (proteins), carry a genetic code (DNA), and reproduce.

Following this definition of life, viruses typically met the latter two tests, but failed the first as they only produced a few structural or protective proteins to encase themselves.  This was a primary justification in classing them as nonliving.  However, recent discoveries have troubled this comfortable notion of the solid boundary between life and nonlife.  First, a variety of parasitic bacteria have been discovered, many of which lack the key protein enzymes needed to survive outside their host -- another bacteria.

Also, giant viruses have been discovered, which in addition to infection related proteins, make a variety of other proteins which help it carry out other processes.  On a level of complexity they can surpass the microbacteria, but they're clearly related to other much smaller viruses.

Now a new piece of evidence supporting that viruses may be somewhat "alive" has been added -- viruses can catch a virus. 

The discovery began more than a decade prior when researchers found a massive parasite in an amoeba from a cooling tower in Bradford, UK.  The little creature was frozen, as it was thought to be run-of-the-mill parasitic bacteria.  However, upon closer inspection, years later the scientists recognized it as a virus, with a gigantic genome, capable of encoding over 900 proteins.  The virus was named Acanthamoeba polyphaga mimivirus (for mimicking microbe).  It was over three times bigger than any previous virus. 

The discovery brought great excitement to some nontraditional biologists who had long believed viruses to be living.  Says Eugene Koonin of the National Center for Biotechnology Information in Bethesda, Maryland, "It was the cause of great excitement in virology.  It crossed the imaginary boundary between viruses and cellular organisms."

Now Professor Koonin, Jean-Michel Claverie, a virologist at the CNRS UPR laboratories and Didier Raoult at CNRS UMR, made an even more shocking discovery.  The team in 2003 discovered an even bigger virus.  They named this one mamavirus.

The shocker came when they found that a smaller virus with just 21 genes was associated with the new mega-virus and was infecting it.  While the main virus infected the amoeba, hijacking the amoeba's enzymes and structure to make a protein factory, the tiny virus, which researchers named "Sputnik" set to work hijacking this factory and making copies of itself.

The result was the mamavirus got more than just a bad cold -- it produced fewer and deformed mamaviruses, effectively making it less infective.  This relationship of a viral parasite sickening a host is one only expected by something living, further evidence that the big viruses might be "alive".

Says Jean-Michel Claverie on the mamavirus, "There’s no doubt this is a living organism.  The fact that it can get sick makes it more alive."

Mr. Koonin adds, "It infects this factory like a phage infects a bacterium.  It’s doing what every parasite can β€” exploiting its host for its own replication."

Intriguingly the little virus has genes similar to those used by the mamavirus and mimiviruses for reproduction.  This leads some researchers to speculate the virus could have been created by a failed reproduction by the big viruses.  This also supports the idea of the big viruses as being alive, as a prevalent theory for the origin of viruses was that they came from misreplicated bacterial DNA.  Further the little virus can transform genes between big viruses, similar to horizontal-gene-transfer in bacteria.

The new finding may have a big impact on global biology.  In plankton blooms genetic sequences have been found similar to those in the big viruses.  These blooms may be teaming with big viruses, which would likely have been destroyed by sample collectors' bacterial filters.  By impacting the life and death of plankton the big viruses could impact not only ocean nutrient cycles, but the global climate itself. 

Curtis Suttle, an expert in marine viruses at the University of British Columbia in Vancouver states, "These viruses could be major players in global systems.  I think ultimately we will find a huge number of novel viruses in the ocean and other places.  It emphasizes how little is known about these organisms β€” and I use that term deliberately."

The full study on the topic of large plankton viruses can be viewed here, while the study on the mamavirus can be viewed here.

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By tmouse on 8/7/2008 11:26:45 AM , Rating: 2
I have never felt strongly one way or another on the topic of whether viruses are "alive" or not (I guess I lean a little more to their being alive). Technically I do not feel these macroviruses are truly infected by the smaller viruses. I have not seen evidence of the merging prior to the infection of a true host by the larger virus, and even if it did it would be unable to do anything until the larger virus infected a host cell. This is just a naturally occurring event similar to the way we use debilitated viruses that require packaging strains for reproduction in the lab. It has also been hypothesized that diseases like AIDS could require "helper" virus co-infection to progress into disease. The use of viruses that are rapidly cleared from mammals like the adenovirus for viral mediated gene transfer as a potential gene therapy agent has been around for a few decades now (although I do have some serious doubts about its long term safety issues). It’s interesting to see it in the wild but in and of itself using viruses to affect other viruses is not entirely a new concept.

RE: Semantics
By geddarkstorm on 8/7/2008 3:47:24 PM , Rating: 2
Yes, there are several cases of retroviruses requiring other retroviruses to infect the same cell at the same time to be able to reproduce, such as one of the avian retroviruses. This is slightly different from what I see, as the mamavirus isn't replicating at the same time as the sputnik virus, but is being used by the latter the same way viruses use the host cell--that is, the sputnik virus is taking over the mamavirus's hijacked replication machinery. It isn't infection in the typical sense, more like competition. The other examples are of piggybacking where one virus is defunct and cannot replicate unless another virus initiates the process, which is more of a mutualism instead of a destructive parasitism between viruses like this new case is. Kinda splitting hairs though as there really isn't much of a difference at all.

Also, the article has one minor error--viruses aren't considered non-living because of the slim amount of proteins they make, but because they cannot replicate on their own at all, and require a host for unit replication. This is different from a parasite which requires a host to provide /the necessary environment/ for living, as well as the necessary nutrients. A virus isn't looking for a habitat or nutrients, instead it cannot make its own factors for replication and must let a cell do the work for it; hence why it's "non-living".

RE: Semantics
By tmouse on 8/8/2008 7:47:09 AM , Rating: 2
I agree with you, I also feel this happens more often than we know. While many viruses are good at preventing multiple infections if two competing viruses infected the same cell then the one with the smallest genome would probably win.
The life debate I still feel is still a bit sketchy. I realize we are talking on the organism level but there are cells we, as biologists, consider "alive" that clearly do not meet the criteria. Mature spermatozoa are nothing more than DNA delivery vehicles and the most extreme case the mature red blood cell which has jettisoned its entire nucleus, clearly has no DNA or any reproductive machinery, yet we never describe them as not being living. Its philosophical hair splitting but we are the ones defining "life" as not requiring assistance from another cell, not of ones own species. In a vague sense it could be thought of as a sexual , cross species mode of reproduction, after all we cannot reproduce without one cell "infecting" another although we keep within our own species.

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