| Mimivirus | |
|---|---|
 | |
| Mimivirus with two satelliteSputnik virophages (arrows)[1] | |
Virus classification | |
| (unranked): | Virus |
| Realm: | Varidnaviria |
| Kingdom: | Bamfordvirae |
| Phylum: | Nucleocytoviricota |
| Class: | Megaviricetes |
| Order: | Imitervirales |
| Family: | Mimiviridae |
| Subfamily: | Megamimivirinae |
| Genus: | Mimivirus |
Mimivirus is a genus ofgiant viruses, in the familyMimiviridae. It is believed thatAmoeba serve as their natural hosts.[2][3] It also refers to a group of phylogenetically related large viruses.[4]
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 apparentGram-staining properties.[5]
Mimivirus has a large and complexgenome compared with most other viruses. Until 2013, when a larger virusPandoravirus was described, it had the largestcapsid diameter of all known viruses.[6]
APMV was discovered accidentally in 1992 within theamoebaAcanthamoeba polyphaga, after which it is named, during research intolegionellosis by researchers from Marseille and Leeds.[7] The virus was observed in aGram stain and mistakenly thought to be aGram-positivebacterium. As a consequence it was namedBradfordcoccus, afterBradford, England, where the amoeba had originated. In 2003, researchers at theUniversité de la Méditerranée inMarseille, France, published a paper inScience identifying the micro-organism as a virus. It was given the name "mimivirus" (for "mimicking microbe") as it resembles a bacterium onGram staining.[8]
The same team that discovered the mimivirus later discovered a slightly larger virus, dubbed themamavirus, and theSputnik virophage that infects it.[9]
Mimivirus has been placed into a viral family by theInternational Committee on Taxonomy of Viruses as a member of theMimiviridae,[10] and has been placed intoGroup I of theBaltimore classification system.[11]
Although not strictly a method of classification, mimivirus joins a group of large viruses known asnucleocytoplasmic 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.[8]
The genusMimivirus contains the following species:[12]
The mimivirus is the fourth-largest virus, after theMegavirus chilensis,Pandoravirus andPithovirus. Mimivirus has acapsid diameter of 400nm. 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 thevirion being casually listed as anywhere between 400 nm and 800 nm, depending on whether total length or capsid diameter is actually quoted.[citation needed]
Its capsid appears hexagonal under anelectron microscope, therefore thecapsid symmetry is icosahedral.[13] It does not appear to possess an outer viral envelope, suggesting that the virus does not exit the host cell byexocytosis.[14]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.[13]
SeveralmRNA transcripts can be recovered from purified virions. Like other NCLDVs, transcripts forDNA polymerase, a capsid protein and a TFII-liketranscription factor were found. However, three distinctaminoacyl tRNA synthetase enzyme transcripts and four unknown mRNA molecules specific to mimivirus were also found. These pre-packaged transcripts can betranslated without viral gene expression and are likely to be necessary to Mimivirus for replication. OtherDNA viruses, such as theHuman cytomegalovirus andHerpes simplex virus type-1, also feature pre-packaged mRNA transcripts.[14]
| Genus | Structure | Symmetry | Capsid | Genomic arrangement | Genomic segmentation |
|---|---|---|---|---|---|
| Mimivirus | Icosahedral | T = 972–1141 orT = 1200 (h = 19 ± 1,k = 19 ± 1) | Linear | Monopartite |
The mimivirus genome is a linear, double-stranded molecule ofDNA with 1,181,404base pairs in length.[15] This makes it one of the largest viral genomes known, outstripping the next-largest virus genome of theCafeteria roenbergensis virus by about 450,000 base pairs. In addition, it is larger than at least 30 cellularclades.[16]
In addition to the large size of the genome, mimivirus possesses an estimated 979 protein-codinggenes.[17] Analysis of its genome revealed the presence of genes not seen in any other viruses, includingaminoacyl 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.[14] Roughly 90% of the genome was of coding capacity, with the other 10% being "junk DNA".[citation needed]
The stages of mimivirus replication are not well known, but as a minimum it is known that mimivirus attaches to achemical receptor on the surface of an amoeba cell and is taken into the cell. Once inside, aneclipse phase begins, in which the virus disappears and all appears normal within the cell. After about 4 hours small accumulations can be seen in areas of the cell. 8 hours after infection many mimivirus virions are clearly visible within the cell. The cellcytoplasm continues to fill with newly synthesised virions, and about 24 hours after initial infection the cell likely bursts open to release the new mimivirus virions.[14]
Little is known[citation needed][when?] 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 usingelectron micrographs of infected cells.[citation needed] 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 asviral factories and allow efficient viral assembly by modifying large areas of the host cell.[citation needed]
| Genus | Host details | Tissue tropism | Entry details | Release details | Replication site | Assembly site | Transmission |
|---|---|---|---|---|---|---|---|
| Mimivirus | Zooplankton | None | Unknown | Unknown | Cytoplasm | Nucleus | Passive diffusion |
Mimivirus may be a causative agent of some forms ofpneumonia; this is based mainly on indirect evidence in the form ofantibodies to the virus discovered in pneumonia patients.[18] However, the classification of mimivirus as apathogen 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,[19] though a mimivirus has been isolated from a Tunisian woman suffering from pneumonia.[20]There is evidence that mimivirus can infectmacrophages.[21]
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