Group I (dsDNA)
A giant virus, also known as a girus, is a very large virus, some of which are larger than typical bacteria. They are giant nucleocytoplasmic large DNA viruses (NCLDVs) that have extremely large genomes compared to other viruses and contain many unique genes not found in other life forms.
While the exact criteria as defined in the scientific literature vary, giant viruses are generally described as viruses having large, pseudo-icosahedral capsids (200 to 400 nanometers) that may be surrounded by a thick (approximately 100 nm) layer of filamentous protein fibers. The viruses' large, double-stranded DNA genomes (300 to 1000 kilobasepairs or larger) encode a large contingent of genes (of the order of 1000 genes). While few giant viruses have been characterized in detail, the most notable examples are the phylogenetically related megavirus and mimivirus — belonging to the Megaviridae and Mimiviridae families, respectively — due to their having the largest capsid diameters of all known viruses.
Viral replication in giant viruses occurs within large circular virus factories located within the cytoplasm of the infected host cell. This is similar to the replication mechanism used by Poxviridae, though whether this mechanism is employed by all giant viruses or only mimivirus and the related mamavirus has yet to be determined. These virion replication factories are themselves subject to infection by the virophage satellite viruses, which inhibit or impair the reproductive capabilities of the complementary virus.
Genetics and evolution
The genomes of giant viruses are the largest known for viruses, and contain genes that encode for important elements of translation machinery, a characteristic that had previously been believed to be indicative of cellular organisms. These genes include multiple genes encoding a number of aminoacyl tRNA synthetases, enzymes that catalyze the esterification of specific amino acids or their precursors to their corresponding cognate tRNAs to form an aminoacyl tRNA that is then used during translation. The presence of four aminoacyl tRNA synthetase encoding genes in mimivirus and mamavirus genomes, both species within the Mimiviridae family, as well as the discovery of seven aminoacyl tRNA synthetase genes, including the four genes present in Mimiviridae, in the megavirus genome provide evidence for a possible scenario in which these large DNA viruses evolved from a shared ancestral cellular genome by means of genome reduction.
Their discovery and subsequent characterization has triggered some debate concerning the evolutionary origins of giant viruses. The two main hypotheses for their origin are that either they evolved from small viruses, picking up DNA from host organisms, or that they evolved from very complicated organisms into the current form which is not self-sufficient for reproduction. What sort of complicated organism giant viruses might have diverged from is also a topic of debate. One proposal is that the origin point actually represents a fourth domain of life, but this is not universally accepted.
Description of the longest giant virus known to date
Three species of giant viruses have been found in three different species of chaetognaths (marine invertebrates). To date, giant viruses known to infect metazoans are unusually rare. In 2018, reanalysis of electron microscopy photographs from the 1980s identified a giant virus infecting Adhesisagitta hispida. These viruses, called Meelsvirus, have a nuclear site of multiplication. The enveloped virions (length: 1.25 μm) are composed of an ovoid nucleocapsid contiguous to a cone terminated by a tail. In 2019, the reanalysis of other previous studies has shown that structures that had been taken in 1967 for bristles present on the surface of the species Spadella cephaloptera, and in 2003, for bacteria infecting Paraspadella gotoi were in fact spindle-shaped giant viruses. The viral species infecting P. gotoi, whose maximum length is 3.1 μm, has been named Klothovirus casanovai [Klotho being the Greek name for one of the three Moirai (Parcae for Latins) whose attribute was a spindle and casanovai in tribute to Pr J.-P. Casanova who devoted a large part of his scientific life to the study of chaetognaths]. The other species has been named Megaklothovirus horridgei (in tribute to the first author of the 1967 article). On a photograph, one of the viruses M. horridgei, although truncated, is 3.9 μm long, corresponding to about twice the length of the bacteria Escherichia coli, largely beating the previous record held by one Pithovirus sibericum extracted from Siberian permafrost (2.5 μm long). The viruses of the family Klothoviridae are enveloped and an intracytoplasmic membrane-like can be observed under a tegument-like structure. Their sites of multiplication are cytoplasmic. The internal part of the viral particles presents electron-dense regions and other very clear; moreover, it contains many ribosomes whose origin is still unknown (cellular, viral or partly viral only). Intracytoplasmic viral particles of M. horridgei already have a spindle shape, unlike those of K. casanovai, suggesting that specific molecules would maintain this structure. To date, the genomes of the three viral species infecting chaetognaths have not yet been sequenced.
Comparison of largest known giant viruses
- Table 1 - Largest giant viruses with complete sequenced genomes
|Giant virus name||Genome Length||Genes||Capsid diameter (nm)||Hair cover||Genbank #|
|Bodo saltans virus||1,385,869||1227 proteins (predicted)||~300||yes (~40 nm)||MF782455|
|Megavirus chilensis||1,259,197||1120 proteins (predicted)||440||yes (75 nm)||JN258408|
|Mamavirus||1,191,693||1023 proteins (predicted)||500||yes (120 nm)||JF801956|
|Mimivirus||1,181,549||979 proteins 39 non-coding||500||yes (120 nm)||NC_014649|
- Table 2 - Specific common features among giant viruses
|Giant virus name||Aminoacyl-tRNA synthetase||Octocoral-like 1MutS||2Stargate||Known virophage||Cytoplasmic virion factory||Host|
|Megavirus chilensis||7 (Tyr, Arg, Met, Cys, Trp, Asn, Ile)||yes||yes||no||yes||Acanthamoeba (Unikonta, Amoebozoa)|
|Mamavirus||4 (Tyr, Arg, Met, Cys)||yes||yes||yes||yes||Acanthamoeba (Unikonta, Amoebozoa)|
|Mimivirus||4 (Tyr, Arg, Met, Cys)||yes||yes||yes||yes||Acanthamoeba (Unikonta, Amoebozoa)|
1Mutator S (MutS) and its homologs are a family of DNA mismatch repair proteins involved in the mismatch repair system that acts to correct point mutations or small insertion/deletion loops produced during DNA replication, increasing the fidelity of replication. 2A stargate is a five-pronged star structure present on the viral capsid forming the portal through which the internal core of the particle is delivered to the host's cytoplasm.
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- head and tail, respectively
- soda lake and deep ocean species of Tupanvirues, respectively
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