title: Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae.
abstract: 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).
authors: Information Génomique et Structurale, Centre National de la Recherche Scientifique-Unité Propre de Recherche 2589, Aix-Marseille University, Institut de Microbiologie de la Méditerranée, Parc Scientifique de Luminy, Case 934, FR-13288 Marseille, France.
authors: Defne
authors: D
authors: Arslan
authors: Matthieu
authors: M
authors: Legendre
authors: Virginie
authors: V
authors: Seltzer
authors: Chantal
authors: C
authors: Abergel
authors: Jean-Michel
authors: JM
authors: Claverie
mesh_terms: D000048:Acanthamoeba / Q000821:virology; D000595:Amino Acid Sequence; D000604:Amino Acyl-tRNA Synthetases / Q000235:genetics; D001483:Base Sequence; D004267:DNA Viruses / Q000145:classification / Q000235:genetics* / Q000648:ultrastructure*; D019143:Evolution, Molecular; D016679:Genome, Viral; D046529:Microscopy, Electron, Transmission; D056686:Mimiviridae / Q000145:classification / Q000235:genetics* / Q000648:ultrastructure*; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D010802:Phylogeny; D012367:RNA, Viral / Q000737:chemistry / Q000235:genetics; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity; D014764:Viral Proteins / Q000235:genetics
chemical_list: D012367:RNA, Viral; D014764:Viral Proteins; D000604:Amino Acyl-tRNA Synthetases