Megagenomoviruses with nearly five megabase genomes and ribosomal proteins

Nucleocytoplasmic large DNA viruses (NCLDVs) infect diverse eukaryotes, but their upper genome size limit is unknown. We report the discovery of "Megagenomoviruses," three giant virus genomes assembled from soil-associated saprobic fungi (Chytridiomycota). With genomes nearing 5 megabases (Mb), these viruses, representing a single species, surpass all known viral genome records. Phylogenetic analysis places Megagenomoviruses within the Klosneuvirinae, related to Yasminevirus (2.1 Mb), but their genomes are massively expanded. This gigantism is characterized by high paralog content suggesting extensive segmental duplications. Strikingly, Megagenomoviruses encode an unprecedented protein biosynthesis capacity, including aminoacyl-tRNA synthetases for all amino acids, 37 tRNAs, 10 translation factors, and ribosomal proteins and ribosome biogenesis factors—rimI, RP-S27Ae, RP-L40e and RP-S30e—which suggests enhanced autonomy. Megagenomoviruses redefine the upper limits of viral genome size and genetic complexity, challenging the fundamental definitions separating viruses from cellular life.