Mamavirus
Encyclopedia
Mamavirus is one of the most complex viruses discovered. It is a virus belonging to the family mimiviridae, which includes mimivirus
, mamavirus, and marseillevirus
The mimiviridae also belong to the family of the nucleocytoplasmic large DNA viruses (NCLDVs), which includes the Poxviridae, the Iridoviridae, the Asfarviridae, and the Phycodnaviridae Like mimivirus, mamavirus is very large, larger than many bacteria
. Mamavirus is actually larger than mimivirus; thus, the name mamavirus, even though mimivirus encodes more than one thousand proteins in its genome of 1.2Mb.
in a cooling tower. The mimiviridae were not discovered until recently because of their size; when filtered the mimiviridae stay with the bacteria which led scientists to believe they were also bacteria. Mimivirus was first isolated in 1992 when scientists were looking for the cause of a pneumonia outbreak in Bradford UK. Due to its size it was named “Bradford coccus” and put in a freezer with scientists thinking it was a bacterium. A decade later, Claverie and Didier Raoult discovered “Bradford coccus” was no bacterium when they tried to digest the cell wall with no success. Deciding to take a different route they looked at it under an electron microscope
. They were surprised to find that it looked like a giant iridovirus, which are icosahedral viruses that infect insects, fish, and frogs. Knowing this paved the way for the discovery of mamavirus because scientists knew to look for other giant viruses.
It was originally isolated from Acanthamoeba polyphaga, but subsequent work has involved Acanthamoeba castellanii.
and a peripheral fiber layer. It contains a linear double-stranded DNA genome
which has a very high coding density that is characteristic of NCLDVs. The mimiviridae contain very similar genomes due to gene duplications, and a fair piece of the genome is associated with functions not previously found in a virus.
machinery, and it packages transcription proteins in its particles. Transcription is believed to occur in the core particles. The core releases viral DNA and forms a cytoplasmic replication factory where DNA replication begins and transcription of late genes occurs. The replication factory forms around the viral core and expands until it occupies a large fraction of the amoeba cell volume. Later stages of the replication cycle involve partially assembled procapsids undergoing DNA packaging.
, a satellite virus. Sputnik contains 21 genes and is very tiny compared to mamavirus; however, it is pretty powerful in its effects on mamavirus. Sputnik cannot replicate in acanthamoeba cells without a simultaneous infection by mamavirus (or mimivirus) so it infects the viral factory mamavirus creates and hijacks it to replicate its own genome. This causes mamavirus to produce fewer viruses that are often deformed and less infective; there is also evidence of a partial thickening of the capsid. The fact that Sputnik can do this suggests that it is a viral parasite, and thus, was named the first virophage. A virophage is like bacteriophage
viruses, which infects and sickens bacteria, but virophages infect viruses. Sputnik contains a circular double-stranded DNA of 18,343 base pairs, and is icosahedral in shape. Of the 21 genes it contains, eight encode proteins that have homologues. Of these eight, three are thought to be derived from mamavirus or mimivirus. This indicates that Sputnik can participate in gene-transfer processes and mediate lateral gene transfer between giant viruses.
Mimivirus
Mimivirus is a viral genus containing a single identified species named Acanthamoeba polyphaga mimivirus , or is a group of phylogenetically related large viruses . In colloquial speech, APMV is more commonly referred to as just “mimivirus”...
, mamavirus, and marseillevirus
Marseillevirus
The Marseillevirus is the prototype of a family of nucleocytoplasmic large DNA viruses of eukaryotes...
The mimiviridae also belong to the family of the nucleocytoplasmic large DNA viruses (NCLDVs), which includes the Poxviridae, the Iridoviridae, the Asfarviridae, and the Phycodnaviridae Like mimivirus, mamavirus is very large, larger than many bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
. Mamavirus is actually larger than mimivirus; thus, the name mamavirus, even though mimivirus encodes more than one thousand proteins in its genome of 1.2Mb.
Discovery
Mamavirus was first reported in September 2008. Like mimivirus, mamavirus was isolated from an amoebaAmoeba
Amoeba is a genus of Protozoa.History=The amoeba was first discovered by August Johann Rösel von Rosenhof in 1757. Early naturalists referred to Amoeba as the Proteus animalcule after the Greek god Proteus, who could change his shape...
in a cooling tower. The mimiviridae were not discovered until recently because of their size; when filtered the mimiviridae stay with the bacteria which led scientists to believe they were also bacteria. Mimivirus was first isolated in 1992 when scientists were looking for the cause of a pneumonia outbreak in Bradford UK. Due to its size it was named “Bradford coccus” and put in a freezer with scientists thinking it was a bacterium. A decade later, Claverie and Didier Raoult discovered “Bradford coccus” was no bacterium when they tried to digest the cell wall with no success. Deciding to take a different route they looked at it under an electron microscope
Electron microscope
An electron microscope is a type of microscope that uses a beam of electrons to illuminate the specimen and produce a magnified image. Electron microscopes have a greater resolving power than a light-powered optical microscope, because electrons have wavelengths about 100,000 times shorter than...
. They were surprised to find that it looked like a giant iridovirus, which are icosahedral viruses that infect insects, fish, and frogs. Knowing this paved the way for the discovery of mamavirus because scientists knew to look for other giant viruses.
It was originally isolated from Acanthamoeba polyphaga, but subsequent work has involved Acanthamoeba castellanii.
Structure and Genome
Mamavirus, like other mimiviridae, is icosahedral with a core capsidCapsid
A capsid is the protein shell of a virus. It consists of several oligomeric structural subunits made of protein called protomers. The observable 3-dimensional morphological subunits, which may or may not correspond to individual proteins, are called capsomeres. The capsid encloses the genetic...
and a peripheral fiber layer. It contains a linear double-stranded DNA genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
which has a very high coding density that is characteristic of NCLDVs. The mimiviridae contain very similar genomes due to gene duplications, and a fair piece of the genome is associated with functions not previously found in a virus.
Replication
Mamavirus possesses its own transcriptionTranscription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
machinery, and it packages transcription proteins in its particles. Transcription is believed to occur in the core particles. The core releases viral DNA and forms a cytoplasmic replication factory where DNA replication begins and transcription of late genes occurs. The replication factory forms around the viral core and expands until it occupies a large fraction of the amoeba cell volume. Later stages of the replication cycle involve partially assembled procapsids undergoing DNA packaging.
Sputnik Virophage
While the mimiviridae were a surprise themselves, mamavirus contained an even bigger surprise. While looking at mamavirus under the electron microscope, Raoult discovered a second, small virus closely associated with mamavirus which was named Sputnik virophageSputnik virophage
Sputnik virophage is a subviral agent that reproduces in amoeba cells that are already infected by a certain helper virus; Sputnik uses the helper virus's machinery for reproduction and inhibits replication of the helper virus.Viruses like Sputnik that depend on co-infection of the host cell by...
, a satellite virus. Sputnik contains 21 genes and is very tiny compared to mamavirus; however, it is pretty powerful in its effects on mamavirus. Sputnik cannot replicate in acanthamoeba cells without a simultaneous infection by mamavirus (or mimivirus) so it infects the viral factory mamavirus creates and hijacks it to replicate its own genome. This causes mamavirus to produce fewer viruses that are often deformed and less infective; there is also evidence of a partial thickening of the capsid. The fact that Sputnik can do this suggests that it is a viral parasite, and thus, was named the first virophage. A virophage is like bacteriophage
Bacteriophage
A bacteriophage is any one of a number of viruses that infect bacteria. They do this by injecting genetic material, which they carry enclosed in an outer protein capsid...
viruses, which infects and sickens bacteria, but virophages infect viruses. Sputnik contains a circular double-stranded DNA of 18,343 base pairs, and is icosahedral in shape. Of the 21 genes it contains, eight encode proteins that have homologues. Of these eight, three are thought to be derived from mamavirus or mimivirus. This indicates that Sputnik can participate in gene-transfer processes and mediate lateral gene transfer between giant viruses.