Isolated as a microorganism in 1992, Mimivirus was mistaken to be a bacterium due to its Gram-positive stain. Study of the Mimivirus within Acanthamoeba polyphaga revealed an icosahedral capsid and consequently identified the microorganism as a virus. The microbe also exhibited a cycle of development that included an eclipse phase, characteristic of viruses. The double-stranded DNA genome of Mimivirus not only lacks universal bacterial genes, but also shares several proteins with known functional attributes and clear homologs with other virus families. The high fraction of mimivirus's open reading frames are without significant similarity to other organisms, a feature also seen in viruses.
Because of its unique size, genome, and morphology, Mimivirus belongs to its own viral family, Mimiviridae . Mimivirus is the largest virus identified to date with a particle diameter of 400 nm and a 1.2 Mbp genome. Though naked, Mimivirus appears to have at least two lipid membranes within its protein capsid as illustrated in cryo-electron micrographs. Its icosahedral structure also appears to have a triangulation number around 1179. From the protein capsid, a multitude of long fibers are projected outwardly. Cryo-electron micrographs also reveal a unique protruding vertex on the Mimivirus capsid. The vertex does not, however, appear to disrupt the symmetry of the virus.
Mimivirus is classified as a nucleocytoplasmic large DNA virus (NCLDV), a group which includes members of Poxviridae , Iridoviridae , Phycodnaviridae , and Asfarviridae .
Pictures: Xiao et al (2005)