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Mimivirus is a genus of , 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.
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.
The same team that discovered the mimivirus later discovered a slightly larger virus, dubbed the mamavirus, and the Sputnik virophage that infects it.
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.
The genus Mimivirus contains the following species:
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
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The mimivirus is the fourth-largest virus, after the Megavirus, Pandoravirus and Pithovirus. Mimivirus has a capsid diameter of 400 Nanometre. Protein filaments measuring 100 nm project from the surface of the capsid, bringing the total length of the virus up to 600 nm. Variation in scientific literature renders these figures as highly approximate, with the "size" of the virion being casually listed as anywhere between 400 nm and 800 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.
Several mRNA transcripts can be recovered from purified virions. Like other NCLDVs, transcripts for DNA polymerase, a capsid protein and a TFII-like transcription factor were found. However, three distinct aminoacyl tRNA synthetase enzyme transcripts and four unknown mRNA molecules specific to mimivirus were also found. These pre-packaged transcripts can be translated without viral gene expression and are likely to be necessary to Mimivirus for replication. Other , such as the Cytomegalovirus and Herpes simplex, also feature pre-packaged mRNA transcripts.
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In addition to the large size of the genome, mimivirus possesses an estimated 979 protein-coding . 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. Roughly 90% of the genome was of coding capacity, with the other 10% being "junk DNA".
Little is known about the details of this replication cycle, most obviously attachment to the cell surface and entry, viral core release, DNA replication, transcription, translation, assembly and release of progeny virions. However, scientists have established the general overview given above using electron micrographs of infected cells. These micrographs show mimivirus capsid assembly in the nucleus, acquisition of an inner lipid membrane via budding from the nucleus, and particles similar to those found in many other viruses, including all NCLDV members. These particles are known in other viruses as viral factories and allow efficient viral assembly by modifying large areas of the host cell.
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