Contributed by guest blogger: Katy Hwang ’12
The discovery of the double stranded DNA (dsDNA) virus, Mimivirus, and the subsequent discovery of related Megavirus confounded the size limits of viral particles and the complexity of viral genomes. They are larger or just as large as some bacteria. Megavirus has a 1,259,197-bp genome. Megavirus contains a genome 6.5% larger than that of Mimivirus. Each of these viruses, classified as Megaviridae, have approximately 979+ proteins, including the first aminoacyl tRNA synthetases (AARS), enzymes that promote translation, found outside of cellular organisms.
Mimivirus infects amoeba; Megavirus natural host is still unknown as it was found in a sea water sample through a campaign on random aquatic environmental sampling off of the coast of Las Cruces, Chile. Megavirus research is conducted in A. castellanii. The virus even reopened the debate on whether or not viruses are alive as Megaviridae have traits that overlap with unicellular organisms such as parasitic bacteria. Phylogenic evidence does not cluster either virus into a prokaryotic clade, but more deeply into a eukaryotic clade. Megavirus is of the archeal type, so it branched out before the radiation of eukaryotes. This provides some insight into the Megaviridae ancestor. So now the question is: from what did these giant viruses originate and how did they evolve?
Megavirus and Mimivirus are similar enough to have unambiguous homologous features, but have also diverged enough on the evolutionarily tree to provide more information on the features of a common ancestor. 23% of the Megavirus genome does not have a counterpart in Mimivirus, but it shares about 77% of its 1120 putative coding sequences with Mimivius. Megavirus and Mimivrius use the same motif to specify early gene expression, the expression pattern of the orthologous genes is conserved globally. Not only do these giant viruses have their own AARS, but they also code for their own DNA repair enzymes that correct damage caused by UV light, ionizing radiation and chemical mutagens. Megavirus also has a DNA photolyase, which is an enzyme that uses light energy to make repairs to DNA and increases resistance to UV radiation.
The author’s hypothesis is that the last Megaviridae common ancestor originated from a cellular organism, where the now current Megaviridae have undergone specific genome reduction events. Mimivirus codes for four AARS and Megavirus codes for seven, four being orthologs to those found in Mimivirus and one of the other AARS that Megavirus codes for is a class-II AARS. This observation that viral AARS are not limited to type-I suggests that independent acquisition of these genes by horizontal gene transfer is unlikely. The ancestor of Megaviridae and Mimivirus most likely started with 20 AARS from a cellular ancestor. It is very unlikely that these seven coding sequences were added, and more likely that there were various lineage specific gene family deletions.
The discovery of the AARS substantiate the claims that the Megaviridae ancestor originated from a cellular organism. What were the driving factors of these genome reduction events? More etiological data is required for further analysis of the evolutionary process of the development into these giant dsDNA viruses. What other giant viruses are still out in the world waiting to be discovered and what can we learn from them?
Katy Hwang is a senior at Vassar College, majoring in biology.