The view that viruses are minute, sneaky agents of devastating disease needs to change, and this revision is being heralded by giant viruses, or, as they are more affectionately known, giruses. Giggle away, I know I did, but the giruses have earned this name given that they may be as big as 400nm in diameter and contain genomes of up 1.2 Mbp (predominantly ds DNA). Indeed, the largest of the giruses, the Mimivirus, was thought to be a coccus when it was discovered in a cooling tower in Bradford, England in 1992.
Another aspect of the giruses that is compelling is there choice of host. Mostly found in aquatic environments, they tend to parasitize protozoans, algae and zooplankton. This has important implications towards ecological balance in said environments. A prime example of this would be the girus EhV whose host of choice is the alga Emiliania huxleyi – a major player in the regulation of flux of carbon dioxide in the oceanic carbon cycles. E. huxleyi is also known to effloresce into expansive blooms that cover the surface of the ocean. Successful EhV infections are important to the clearance of these blooms, and also result in the release of a substance known at dimethylsulphonioproprionate (DMSP). DMSP is a common metabolite taken up by marine bacteria, and they break it down to acrylate and dimethylsulphide. The dimethylsulphide, once released into the environment, prompts the formation of clouds, and is thus a bringer of rain. Yes, a virus that makes rain. I just wrote that, and it makes me all sorts of happy.
As rivetting as that is, to this writer, the true enigma of giruses lies within the many curios that their genomes contain. Laconically put, the diversity of genes (and thusly gene products) is immense, but a common theme across the board (from the Poxviridae- like Mimiviruses to the Iridoviridae cousins that are the Phycodnaviridae) is the presence of genes that encode for structural features, a DNA polymerase, an ATP-ase and some transcriptional machinery. While the first three of this list are regular enough, why bear transcription machinery, though? Could it be an elaborate infective strategy that tunes the infected to the virus’s choice of metabolic music? Or, could it be a remnant of a full protein synthesis apparatus (for the Mimivirus does contain genes that encode an amino acyl t-RNA synthase) that was evolutionally reduced because (as we are wont to say in science) Something Happened that tipped the evolutionary scale towards this ancient entity becoming a virus?
I prefer the latter theory. It is my belief that the giant viruses, primeval as they are, inform the presence of a missing link between the molecular evolution of life itself. This contention is further strengthened when one notes that EhV encodes gene products that are putatively involved in sphingolipid biosynthesis. But viruses don’t carry out metabolism, one would be quick to say. Working from general principles, they do not, but these! These do! The trouble (and I use this descriptor carefully) arises from the fact that our general principles are based on what we now know to be viruses—those that dance along the border that divides the realm of the living from the non-living. These giant, primeval viruses bespeak a stage in evolution that lies between cells and viruses.
Inscrutable as they are, giruses are poised to answer a lot of the questions of where Life began, and maybe even predict where it is going. I, for one, shall be watching closely!
References: Van Etten J.L. (2011).“Another Really, Really Big Virus.” Viruses. 3.1: 32-46.
I also recommend the following short piece about an ingenious interaction between Sputnik and the Mamavirus. It is a significantly more devious take on the rather wretched tale of the Dependovirus: