I make no secret of my admiration for Mimivirus, the largest virus known. I make quite a play of what Mimivirus tells us about the nature of viruses and virus evolution in the new edition of Principles of Molecular Virology:
But of course, as soon as you publish a printed textbook, it’s out of date (that’s why I write this blog). And so it proved this week when Mimivirus was knocked off it’s throne by the latest Girus to come along – the even bigger OMGItsSoHugeItBlocksOutTheSun virus. Well, actually, they called it Megavirus (but you get the general idea). With a genome of 1.26 million base pairs of DNA (megabases), this is now the largest virus known (until we discover an even bigger one).
So what do these monsters tell us about viruses? Probably quite a lot, as this excellent Wired article describes.
Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae. PNAS USA 10 Oct 2011. DOI: 10.1073/pnas.1110889108
Mimivirus, a DNA virus infecting acanthamoeba, was for a long time the largest known virus both in terms of particle size and gene content. Its genome encodes 979 proteins, including the first four aminoacyl tRNA synthetases (ArgRS, CysRS, MetRS, and TyrRS) ever found outside of cellular organisms. The discovery that Mimivirus encoded trademark cellular functions prompted a wealth of theoretical studies revisiting the concept of virus and associated large DNA viruses with the emergence of early eukaryotes. However, the evolutionary significance of these unique features remained impossible to assess in absence of a Mimivirus relative exhibiting a suitable evolutionary divergence. Here, we present Megavirus chilensis, a giant virus isolated off the coast of Chile, but capable of replicating in fresh water acanthamoeba. Its 1,259,197-bp genome is the largest viral genome fully sequenced so far. It encodes 1,120 putative proteins, of which 258 (23%) have no Mimivirus homologs. The 594 Megavirus/Mimivirus orthologs share an average of 50% of identical residues. Despite this divergence, Megavirus retained all of the genomic features characteristic of Mimivirus, including its cellular-like genes. Moreover, Megavirus exhibits three additional aminoacyl-tRNA synthetase genes (IleRS, TrpRS, and AsnRS) adding strong support to the previous suggestion that the Mimivirus/Megavirus lineage evolved from an ancestral cellular genome by reductive evolution. The main differences in gene content between Mimivirus and Megavirus genomes are due to (i) lineages specific gains or losses of genes, (ii) lineage specific gene family expansion or deletion, and (iii) the insertion/migration of mobile elements (intron, intein).