<!--thumb| This does not show the stargate, but seems to show a bipartite genom, but its monopartite (unsegmented)-->

thumb|upright=1.5|Schematic drawing of a virion of genus Mimivirus (cross section and side view) showing filaments ("hairs") and stargate (downside)

Mimivirus is a genus of giant viruses, in the family Mimiviridae. It is believed that Amoeba serve as their natural hosts. It also refers to a group of phylogenetically related large viruses<!--, designated usually "MimiN"-- MimiN cannot be found in reference specified, nor elsewhere in internet-->.

In colloquial speech, APMV is more commonly referred to as just "mimivirus". Mimivirus, short for "mimicking microbe", is so called to reflect its large size and apparent Gram-staining properties.

Mimivirus has a large and complex genome compared with most other viruses. Until 2013, when a larger virus Pandoravirus was described, it had the largest capsid diameter of all known viruses.

History

APMV was discovered accidentally in 1992 within the amoeba Acanthamoeba polyphaga, after which it is named, during research into legionellosis by researchers from Marseille and Leeds. The virus was observed in a Gram stain and mistakenly thought to be a Gram-positive bacterium. As a consequence it was named Bradfordcoccus, after Bradford, England, where the amoeba had originated. In 2003, researchers at the Université de la Méditerranée in Marseille, France, published a paper in Science identifying the micro-organism as a virus. It was given the name "mimivirus" (for "mimicking microbe") as it resembles a bacterium on Gram staining.

The same team that discovered the mimivirus later discovered a slightly larger virus, dubbed the mamavirus, and the Sputnik virophage that infects it.

Classification

Mimivirus has been placed into a viral family by the International Committee on Taxonomy of Viruses as a member of the Mimiviridae, and has been placed into Group I of the Baltimore classification system.

Although not strictly a method of classification, mimivirus joins a group of large viruses known as nucleocytoplasmic large DNA viruses (NCLDV). They are all large viruses which share both molecular characteristics and large genomes. The mimivirus genome also possesses 21 genes encoding homologs to proteins which are seen to be highly conserved in the majority of NCLDVs, and further work suggests that mimivirus is an early divergent of the general NCLDV group.

  • Mimivirus bradfordmassiliense
  • Mimivirus lagoaense

Structure

[[File:Mimivirus fibers - journal.pbio.1000092.g007.png|thumb|right|

A: AFM image of several surface fibers attached to a common central feature.

B: AFM image of two detached surface fibers of Mimivirus.

C: CryoEM image of a Mimivirus after partial digestion of fibrils with Bromelain.

D: AFM image of internal fibers of Mimivirus

]]

The mimivirus is the fourth-largest virus, after the Megavirus chilensis, Pandoravirus and Pithovirus. Mimivirus has a capsid diameter of 400 nm. Protein filaments measuring 100&nbsp;nm project from the surface of the capsid, bringing the total length of the virus up to 600&nbsp;nm. Variation in scientific literature renders these figures as highly approximate, with the "size" of the virion being casually listed as anywhere between 400&nbsp;nm and 800&nbsp;nm, depending on whether total length or capsid diameter is actually quoted.

Its capsid appears hexagonal under an electron microscope, therefore the capsid symmetry is icosahedral. It does not appear to possess an outer viral envelope, suggesting that the virus does not exit the host cell by exocytosis.

Mimivirus shares several morphological characteristics with all members of the NCLDV group of viruses. The condensed central core of the virion appears as a dark region under the electron microscope. The large genome of the virus resides within this area. An internal lipid layer surrounding the central core is present in all other NCLDV viruses, so this features may also be present in mimivirus. This makes it one of the largest viral genomes known, outstripping the next-largest virus genome of the Cafeteria roenbergensis virus by about 450,000 base pairs. In addition, it is larger than at least 30 cellular clades.

In addition to the large size of the genome, mimivirus possesses an estimated 979 protein-coding genes. Analysis of its genome revealed the presence of genes not seen in any other viruses, including aminoacyl tRNA synthetases, and other genes previously thought only to be encoded by cellular organisms. Like other large DNA viruses, mimivirus contains several genes for sugar, lipid and amino acid metabolism, as well as some metabolic genes not found in any other virus. However, the classification of mimivirus as a pathogen is tenuous at present as there have been only a couple of papers published potentially linking mimivirus to actual cases of pneumonia. A significant fraction of pneumonia cases are of unknown cause, though a mimivirus has been isolated from a Tunisian woman suffering from pneumonia.

There is evidence that mimivirus can infect macrophages.

See also

  • Cafeteria roenbergensis virus—a giant marine virus
  • Mamavirus
  • Marseillevirus—another giant virus
  • Megavirus—another giant virus
  • Mycoplasma genitalium—one of the smallest bacteria
  • Nanoarchaeum equitans—smallest known independent cell
  • Nanobacterium
  • Nanobe
  • Non-cellular life
  • Pandoravirus
  • Pithovirus—the largest known virus
  • Parvovirus—smallest known viruses
  • Pelagibacter ubique—possesses one of the smallest bacterial genomes
  • Virophage—a virus that requires the host cell to be co-infected with a giant virus
  • The Giant Virus Finder is a software tool that identifies giant viruses in environmental Metagenomes.

References

Further reading

  • Viralzone: Mimiviridae
  • International Committee on Taxonomy of Viruses (ICTV) picture gallery—images of mimivirus.
  • The webpage for Mimivirus
  • Radiolab.org Shrink on the discovery of Mimivirus Thursday, July 30, 2015 - 08:54 PM.