Discovery
APMV was accidentally discovered in 1992 within theamoeba Acanthamoeba polifaga, which gives it its name, during an investigation into legionellosis carried out by researchers fromMarseillesand Leeds. The virus was seen on a Gram stain and was wrongly assumed to be a Gram-positive bacterium. Accordingly, he was appointedbradfordcoccus, after the district of origin of the amoeba inbradford, England. In 2003, researchers from the University of the Mediterranean in Marseille, France, published an article inScienceidentify the microorganism as a virus. It was given the name "Mimivirus" (for "mimicking microbe"), as it resembles a bacterium on Gram stain.
The same team that discovered the mimivirus later discovered a slightly larger virus, calledmamavirus, and the virophage Sputnik that infects it.
Classification
Mimivirus was placed in a viral family by theInternational Committee on Taxonomy of Virusesas a member ofMimiviridae, and was placed in Group I of theBaltimore Classificationsystem.
While not strictly a classification method, Mimivirus joins a group of large viruses known as large nucleocytoplasmic viruses.DNA viruses(NCLDV). They are all large viruses that share molecular features and large genomes. The mimivirus genome also has 21genesthey encode homologs for proteins that are considered highly conserved in most NCLDVs, and subsequent work suggests that the mimivirus is an early divergence from the general group of NCLDVs.
Structure
Mimivirus is the fourth largest virus, preceded by the recently discoveredMegavirus chilensis,virus pandoramithe pitovirus. Mimivirus has acapsid400 nm diameter. Protein strands measuring 100 nm protrude from the capsid surface, bringing the total length of the virus to 600 nm. Variation in the scientific literature makes these numbers very approximate, and virion "size" is casually listed between 400 nm and 800 nm, depending on whether total length or capsid diameter is actually being quoted.
Its capsid appears hexagonal under aelectronic microscope, so the symmetry of the capsid is icosahedral. It does not appear to have an external viral envelope, suggesting that the virus does not exit the host cell byexocytose.
Mimiviruses share several morphological features with all members of the NCLDV group of viruses. The central condensed core of the virion appears as a dark region under the electron microscope. The large genome of the virus resides in this area. An inner lipid layer surrounding the central core is present in all other NCLDV viruses, so these features may also be present in mimiviruses.
Various mRNA transcripts can be recovered from purified virions. Like other NCLDVs, transcripts ofDNA polymerase, a capsid protein and a type of TFIItranscription factorThey were found. However, three differentaminoacyl tRNA synthetaseEnzymatic transcripts and four unknown mimivirus-specific mRNA molecules were also found. These prepackaged transcripts can be translated without viral gene expression and will likely be required for Mimivirus replication. OtherADNviruses, like thehuman cytomegalovirusmiHerpes simplex virus type 1, also have prepackaged mRNA transcripts.
genome
The mimivirus genome is a linear double-stranded DNA molecule 1,181,404 base pairs long. This makes it one of the largest known viral genomes, dwarfing the next largest viral genome in the world.cafeteria roenbergensisvirus by about 450,000 base pairs. Also, it is larger than at least 30 mobile phones.clados.
In addition to the large genome size, the mimivirus has about 979 protein-coding genes, far exceeding the minimum of 4 genes required for virus existence (cf MS2and Qβ virus). Analysis of its genome revealed the presence of genes not seen in any other virus, includingaminoacyl tRNA synthetases, and other genes previously thought to be only encoded by cellular organisms. Like other large DNA viruses, the mimivirus contains several genes for sugar, lipids, andamino acidsmetabolism, as well as some metabolic genes not found in any other virus. Approximately 90% of the genome was capable of coding, the other 10% was "junk DNA".
replication
The replication stages of mimiviruses are not well understood, but at a minimum, mimiviruses are known to adhere to achemical receptoron the surface of an amoeba cell and enters the cell. Once inside, oneeclipse phasebegins, in which the virus disappears and everything seems normal inside the cell. After approximately 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 cellcytoplasmit continues to fill with newly synthesized virions, and about 24 hours after the initial infection, the cell probably opens to release the new mimivirus virions.
Little is known about the details of this replication cycle, most obviously cell surface attachment and entry, release of the viral core, DNA replication, transcription, translation, assembly, and release of virions from the progeny. However, scientists have established the general description given above using electron micrographs of infected cells. These micrographs show mimivirus capsid assembly in the nucleus, the acquisition of an inner lipid membrane by budding from the nucleus, and particles similar to those found in many other viruses, including all members of NCLDV. These particles are known in other viruses asviral factoriesand allow efficient viral assembly, modifying large areas of the host cell.
pathogenicity
Mimivirus may be a causative agent of some forms ofpneumonia; this is mainly based on indirect evidence in the form of antibodies to the virus discovered in patients with pneumonia. However, the classification of mimivirus as apathogenis tenuous at this time, as there are only a few published papers potentially linking mimivirus to actual cases of pneumonia. A significant fraction of pneumonia cases are of unknown etiology, although a mimivirus has been isolated from a Tunisian woman with pneumonia.
Implications for the definition of "life"
The mimivirus has many characteristics that place it on the border between living and non-living. It is as large as many bacterial species, such asRickettsia conoriimiTropheryma whipplei, has a genomic size comparable to that of severalbacteria, including the above, and codes for products not previously believed to be coded with viruses (including a type ofcollagen). In addition, the mimivirus has genes that encodenucleotideand amino acid synthesis, which is lacking even in some small obligate intracellular bacteria. However, they lack genes for ribosomal proteins, making the mimivirus dependent on a host cell for protein translation and energy metabolism.
Since its lineage is very old and may have arisen before cellular organisms,Mimiviruscontributed to the debate on the origin of life. Some genes that encode unique traits forMimivirus, including those encoding the capsid, have been conserved in a variety of viruses that infect organisms of all types.domains. This has been used to suggest thatMimivirusit is related to a type of DNA virus that arose before cellular organisms and played a key role in the development of all life on Earth. An alternative hypothesis is that there were three distinct types of DNA viruses involved in the generation of the three known domains of life: eukarya,you archand bacteria. It has been suggested thatMimivirusand similar types are remnants of a "fourth domain" of life, and this other giant virus may represent other ancient domains.
However, mimivirus lacks the following characteristics, which are part of many conventional definitions of life: