Mimiviruses and Marseilleviruses, the Largest Known Viruses

Abstract

Acanthamoeba polyphaga mimivirus was discovered in 2003 but was isolated two decades ago during the investigation of a pneumonia outbreak, while trying to isolate Legionella ‐like bacterial pathogens that infect and survive in amoebae. Acanthamoeba polyphaga marseillevirus was isolated five years thereafter. These two viruses are the founding members of two new viral families whose members are the largest known viruses based on the sizes of the capsid and the genome. Beyond being giant, mimiviruses and marseilleviruses exhibit several unexpected and remarkable features. Their genomes encode proteins involved in translation, these viruses harbour messenger ribonucleic acid in addition to their double‐stranded deoxyribonucleic acid (DNA) genome, and their gene repertoire indicates that they have a common ancestral origin with other large DNA viruses. Finally, mimiviruses and marseilleviruses are likely to be common in our biosphere and have begun to be isolated from humans. Key Concepts: Mimiviruses and marseilleviruses are giant viruses that infect phagocytic protists (Acanthamoeba spp. in most cases). Mimiviruses are the largest known viruses, and their genomes are the largest among viruses and are larger than those of the smallest cellular life. Mimiviruses and marseilleviruses differ greatly from ‘classical’ viruses in several respects. Mimiviruses and marseilleviruses are linked by phylogeny based on certain genes and their core genomes to the members of the five families of nucleocytoplasmic large DNA viruses; these seven families were recently proposed to be assigned to a new viral order, Megavirales. The mosaic gene repertoires of mimiviruses and marseilleviruses are likely linked to their sympatric lifestyle within phagocytic protistan grazers. Mimiviruses and marseilleviruses are common in our biosphere. Mimiviruses may cause pneumonia in humans.