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severalog

No big deal. Just some big viruses.

J. Harry Caufield

Searching PubMed for the keyword "giant virus" always provides some fun results. That's not what I did today - though I do recommend trying it sometime - as the recent reports about oversized viruses have been spreading like some kind of very small causative agent of infectious disease.

Here's the first one: the isolation of a Mimivirus from a patient with pneumonia. The particular viral isolate is more than 550 nanometers wide and has a 1.23 megabase genome. For reference, that's huge. For a better reference, that's about the width of an average E. coli cell and a genome in the same size range as many of the more genetically streamlined bacteria (it's more than twice as large as the tiny Mycoplasma genitalium genome, though that's about as minimal as known bacterial genomes get). Most viruses we know of aren't quite this massive, though Mimiviruses and other record-holders for viral size share the characteristic of infecting Acanthamoeba polyphaga amoebae. So if this new mimivirus infects amoebae, is it pathogenic to humans as well? The authors of this paper seem to think so. As always, further viral isolates will be necessary. (The actual paper is right on the other side of this concrete paywall.) 

If you thought that mimivirus was big and/or had a silly name, check out the Pandoraviruses. These viral isolates average more than 700 nanometers in diameter and bear genomes of, in at least one case, more than 2 megabases. They were found in sediments and mud where amoebae are plentiful. There are some hyperbolic news reports out there about these new viruses already so I'll just pick some interesting bits out of the paper itself:
Unlike eukaryotic DNA viruses and phages, which first synthesize and then fill their capsids, the tegument and internal compartment of the Pandoravirus particles are synthesized simultaneously, in a manner suggestive of knitting, until the particles are fully formed and closed.
Knitting viruses. Don't tell Pinterest. Not yet.

The high percentage (93%) of CDSs without recognizable homolog (ORFans), the alien morphological features displayed by P. salinus, and its atypical replication process raised the concern that the translation of its genes into proteins might not obey the standard genetic code, hence obscuring potential sequence similarities.
The authors are trying to say that these viruses are almost suspiciously strange. It's not uncommon to see large chunks of viral genome sequences that don't look like any known sequence, but when you're talking about genomes larger than many bacterial ones then this becomes a sizable reservoir of new, uncharacterized genes and proteins. They may be more familiar than we can initially tell.

...their DNA polymerase does cluster with those of other giant DNA viruses, suggesting the controversial existence of a fourth domain of life ... The absence of Pandoravirus-like sequences from the rapidly growing environmental metagenomic databases suggests either that they are rare or that their ecological niche has never been prospected. However, the screening of the literature on Acanthamoeba parasites does reveal that Pandoravirus-like particles had been observed 13 years ago ... although not interpreted as viruses. 
So these viruses aren't totally alien. They've been around for at least 13 years! Probably even longer,* though exactly how much longer may determine how controversial that claim about the "fourth domain of life" becomes.

Citations follow.

Saadi, H., Pagnier, I., Colson, P., Cherif, J. K., Beji, M., Boughalmi, M., … Raoult, D. (2013). First Isolation of Mimivirus in a Patient With Pneumonia. Clinical infectious diseases: an official publication of the Infectious Diseases Society of America. doi:10.1093/cid/cit354.

Philippe, N., Legendre, M., Doutre, G., Coute, Y., Poirot, O., Lescot, M., … Abergel, C. (2013). Pandoraviruses: Amoeba Viruses with Genomes Up to 2.5 Mb Reaching That of Parasitic Eukaryotes. Science, 341(6143), 281–286. doi:10.1126/science.1239181.


* Some varieties of amoebae may have been around as long ago as the Neoproterozoic Era, or between 1,000 and 541 million years ago. If there were amoebae then viruses with amoebic hosts may have also been present.