Mimivirus

Abstract

Acanthamoeba polyphaga mimivirus is the largest known virus both in terms of particle size (0.75 μm in diameter) and genome complexity (1.2 million bp encoding 911 proteins). Easily visible under the light microscope following Gram coloration, mimivirus was initially mistaken for an intracellular parasitic bacterium. It was serendipitously discovered in 1992 during a routine investigation for legionella within the amoeba colonizing the water of a cooling tower, in Bradford, UK. In addition to its established tropism for Acanthamoeba cells, mimivirus might be pathogenic to human causing pneumonia. Mimivirus is a paradoxical microorganism. On the one hand, it exhibits the morphological and genomic features expected from a nucleocytoplasmic large DNA virus, defining a new family, the Mimiviridae. Its genome contains a large number of genes identified for the first time in a virus and encodes key biochemical functions previously thought to be the trademark of cellular organisms, namely components of the protein translation machinery. Mimivirus exhibits a clear phylogenetic relationship with large phytoplankton viruses ( Phycodnaviridae), suggesting that additional mimivirus relatives remain to be discovered within the largely unexplored populations of marine protists. The exceptional features of mimivirus lend some support to various evolutionary hypotheses proposing that ancestral DNA viruses might have played a central role in the emergence of the eukaryotic lineage.