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While the exact criteria as defined in the scientific literature vary, giant viruses are generally described as viruses having large, pseudo-[[Capsid#Icosahedral|icosahedral capsids]] (200 to 400 nanometers)<ref>{{cite journal | vauthors = Xiao C, Fischer MG, Bolotaulo DM, Ulloa-Rondeau N, Avila GA, Suttle CA | title = Cryo-EM reconstruction of the Cafeteria roenbergensis virus capsid suggests novel assembly pathway for giant viruses | journal = Scientific Reports | volume = 7 | issue = 5484 | date = 14 July 2017 | page = 5484 | pmid = 28710447 | pmc = 5511168 | doi = 10.1038/s41598-017-05824-w | bibcode = 2017NatSR...7.5484X }}</ref> 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).<ref name=AmSci-099-4/><ref name=Comm-005-1/> While few giant viruses have been characterized in detail, the most notable examples are the phylogenetically related [[mimivirus]] and [[megavirus]]—both belonging to the ''[[Mimiviridae]]'' (aka ''Megaviridae'') family, due to their having the largest capsid diameters of all known viruses.<ref name=AmSci-099-4/><ref name=Comm-005-1>{{cite journal | vauthors = Legendre M, Arslan D, Abergel C, Claverie JM | title = Genomics of Megavirus and the elusive fourth domain of Life | journal = Communicative & Integrative Biology | volume = 5 | issue = 1 | pages = 102–6 | date = January 2012 | pmid = 22482024 | pmc = 3291303 | doi = 10.4161/cib.18624 }}</ref>Giant viruses from the deep ocean, terrestrial sources, and human patients contain genes encoding [[Cytochrome P450|cytochrome P450 (CYP; P450) enzymes]]. The origin of these P450 genes in giant viruses remains unknown but may have been acquired from an ancient host.<ref>{{cite journal|display-authors=6|vauthors=Lamb DC, Follmer AH, Goldstone JV, Nelson DR, Warrilow AG, Price CL, True MY, Kelly SL, Poulos TL, Stegeman JJ|date=June 2019|title=On the occurrence of cytochrome P450 in viruses|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=116|issue=25|pages=12343–12352|doi=10.1073/pnas.1901080116|pmc=6589655|pmid=31167942|doi-access=free}}</ref> |
While the exact criteria as defined in the scientific literature vary, giant viruses are generally described as viruses having large, pseudo-[[Capsid#Icosahedral|icosahedral capsids]] (200 to 400 nanometers)<ref>{{cite journal | vauthors = Xiao C, Fischer MG, Bolotaulo DM, Ulloa-Rondeau N, Avila GA, Suttle CA | title = Cryo-EM reconstruction of the Cafeteria roenbergensis virus capsid suggests novel assembly pathway for giant viruses | journal = Scientific Reports | volume = 7 | issue = 5484 | date = 14 July 2017 | page = 5484 | pmid = 28710447 | pmc = 5511168 | doi = 10.1038/s41598-017-05824-w | bibcode = 2017NatSR...7.5484X }}</ref> 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).<ref name=AmSci-099-4/><ref name=Comm-005-1/> While few giant viruses have been characterized in detail, the most notable examples are the phylogenetically related [[mimivirus]] and [[megavirus]]—both belonging to the ''[[Mimiviridae]]'' (aka ''Megaviridae'') family, due to their having the largest capsid diameters of all known viruses.<ref name=AmSci-099-4/><ref name=Comm-005-1>{{cite journal | vauthors = Legendre M, Arslan D, Abergel C, Claverie JM | title = Genomics of Megavirus and the elusive fourth domain of Life | journal = Communicative & Integrative Biology | volume = 5 | issue = 1 | pages = 102–6 | date = January 2012 | pmid = 22482024 | pmc = 3291303 | doi = 10.4161/cib.18624 }}</ref>Giant viruses from the deep ocean, terrestrial sources, and human patients contain genes encoding [[Cytochrome P450|cytochrome P450 (CYP; P450) enzymes]]. The origin of these P450 genes in giant viruses remains unknown but may have been acquired from an ancient host.<ref>{{cite journal|display-authors=6|vauthors=Lamb DC, Follmer AH, Goldstone JV, Nelson DR, Warrilow AG, Price CL, True MY, Kelly SL, Poulos TL, Stegeman JJ|date=June 2019|title=On the occurrence of cytochrome P450 in viruses|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=116|issue=25|pages=12343–12352|doi=10.1073/pnas.1901080116|pmc=6589655|pmid=31167942|doi-access=free}}</ref> |
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The genomes of many giant viruses encode many unusual genes that are not found in other viruses, including genes involved in [[glycolysis]] and the [[TCA cycle]]<ref>{{cite journal | vauthors = Moniruzzaman M, Martinez-Gutierrez CA, Weinheimer AR, Aylward FO | title = Dynamic genome evolution and complex virocell metabolism of globally-distributed giant viruses | journal = Nature Communications | volume = 11 | issue = 1710 | date = 2020 | doi = 10.1038/s41467-020-15507-2 | pmid = 32249765 }}</ref>, fermentation<ref>{{cite journal | vauthors = Schvarcz CR, Steward GF | title = A giant virus infecting green algae encodes key fermentation genes | journal = Virology | date = 2018 | doi = 10.1016/j.virol.2018.03.010 | pmid = 29649682 }}</ref>, and the [[cytoskeleton]] |
The genomes of many giant viruses encode many unusual genes that are not found in other viruses, including genes involved in [[glycolysis]] and the [[TCA cycle]]<ref>{{cite journal | vauthors = Moniruzzaman M, Martinez-Gutierrez CA, Weinheimer AR, Aylward FO | title = Dynamic genome evolution and complex virocell metabolism of globally-distributed giant viruses | journal = Nature Communications | volume = 11 | issue = 1710 | date = 2020 | doi = 10.1038/s41467-020-15507-2 | pmid = 32249765 }}</ref>, fermentation<ref>{{cite journal | vauthors = Schvarcz CR, Steward GF | title = A giant virus infecting green algae encodes key fermentation genes | journal = Virology | date = 2018 | doi = 10.1016/j.virol.2018.03.010 | pmid = 29649682 }}</ref>, and the [[cytoskeleton]]<ref>{{cite journal | vauthors = Da Cunha V, Gaia M, Ogata H, Jaillon O, Delmont TO, Patrick Forterre P | title = Giant viruses encode novel types of actins possibly related to the origin of eukaryotic actin: the viractins | journal = bioRxiv | doi = 10.1101/2020.06.16.150565 }}</ref><ref>{{cite journal | vauthors = Ha AD, Moniruzzaman M, Aylward FO | title = High Transcriptional Activity and Diverse Functional Repertoires of Hundreds of Giant Viruses in a Coastal Marine System | journal = mSystems | volume = 6 | issue = 4 | date = 2021 | doi = 10.1128/mSystems.00293-21 | pmid = 34254826}}</ref><ref>{{cite journal | vauthors = Kijima S, Delmont TO, Miyazaki U, Gaia M, Endo H, Ogata H | title = Discovery of Viral Myosin Genes With Complex Evolutionary History Within Plankton | journal = bioRxiv | doi = 10.3389/fmicb.2021.683294 | pmid = 34163457}}</ref>. |
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[[File:CroV TEM.jpg|thumb|Cryo-EM images of the giant viruses [[CroV]] and [[APMV]]. (A) Cryo-electron micrograph of four CroV particles. (B) Single CroV particle with concave core depression (white arrow). (C) Single APMV particle. Scale bars in (A–C) represent 2,000 Å. ]] |
[[File:CroV TEM.jpg|thumb|Cryo-EM images of the giant viruses [[CroV]] and [[APMV]]. (A) Cryo-electron micrograph of four CroV particles. (B) Single CroV particle with concave core depression (white arrow). (C) Single APMV particle. Scale bars in (A–C) represent 2,000 Å. ]] |
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The genomes of giant viruses are the largest known for viruses, and contain genes that encode for important elements of [[Translation (biology)|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 synthetase]]s, enzymes that catalyze the [[esterification]] of specific amino acids or their precursors to their corresponding cognate [[tRNA]]s to form an [[aminoacyl tRNA]] that is then used during translation.<ref name=Comm-005-1/> 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 size#Genome reduction|genome reduction]].<ref name=Comm-005-1/> |
The genomes of giant viruses are the largest known for viruses, and contain genes that encode for important elements of [[Translation (biology)|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 synthetase]]s, enzymes that catalyze the [[esterification]] of specific amino acids or their precursors to their corresponding cognate [[tRNA]]s to form an [[aminoacyl tRNA]] that is then used during translation.<ref name=Comm-005-1/> 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 size#Genome reduction|genome reduction]].<ref name=Comm-005-1/> |
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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.<ref>{{cite news | first = Rae Ellen | last = Bichell | name-list-style = vanc | title = In Giant Virus Genes, Hints About Their Mysterious Origin | url = https://www.npr.org/sections/health-shots/2017/04/06/522478901/in-giant-virus-genes-hints-about-their-mysterious-origin | work = All Things Considered }}</ref> 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 [[Three-domain system|domain]] of life,<ref name=Comm-005-1/> but this has been largely discounted.<ref>{{cite journal | vauthors = Schulz F, Yutin N, Ivanova NN, Ortega DR, Lee TK, Vierheilig J, Daims H, Horn M, Wagner M, Jensen GJ, Kyrpides NC, Koonin EV, Woyke T | title = Giant viruses with an expanded complement of translation system components | journal = Science | volume = 356 | issue = 6333 | pages = 82–85 | date = April 2017 | pmid = 28386012 | doi = 10.1126/science.aal4657 | bibcode = 2017Sci...356...82S | s2cid = 206655792 | url = https://escholarship.org/content/qt0kf9t6gn/qt0kf9t6gn.pdf?t=oruwia | doi-access = free }}</ref><ref>{{cite journal | vauthors = Bäckström D, Yutin N, Jørgensen SL, Dharamshi J, Homa F, Zaremba-Niedwiedzka K, Spang A, Wolf YI, Koonin EV, Ettema TJ | title = Virus Genomes from Deep Sea Sediments Expand the Ocean Megavirome and Support Independent Origins of Viral Gigantism | journal = mBio | volume = 10 | issue = 2 |pages=e02497-02418 | date = March 2019 | doi = 10.1128/mBio.02497-18 | pmid = 30837339 | pmc = 6401483 }}</ref><ref>{{cite journal | vauthors = Yutin N, Wolf Y, Koonin EV | title = Origin of giant viruses from smaller DNA viruses not from a fourth domain of cellular life | journal = Virology | volume = 466–467 | issue = 2014 |pages=38-52 | date = 2014 | doi = 10.1016/j.virol.2014.06.032 | |
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.<ref>{{cite news | first = Rae Ellen | last = Bichell | name-list-style = vanc | title = In Giant Virus Genes, Hints About Their Mysterious Origin | url = https://www.npr.org/sections/health-shots/2017/04/06/522478901/in-giant-virus-genes-hints-about-their-mysterious-origin | work = All Things Considered }}</ref> 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 [[Three-domain system|domain]] of life,<ref name=Comm-005-1/> but this has been largely discounted.<ref>{{cite journal | vauthors = Schulz F, Yutin N, Ivanova NN, Ortega DR, Lee TK, Vierheilig J, Daims H, Horn M, Wagner M, Jensen GJ, Kyrpides NC, Koonin EV, Woyke T | title = Giant viruses with an expanded complement of translation system components | journal = Science | volume = 356 | issue = 6333 | pages = 82–85 | date = April 2017 | pmid = 28386012 | doi = 10.1126/science.aal4657 | bibcode = 2017Sci...356...82S | s2cid = 206655792 | url = https://escholarship.org/content/qt0kf9t6gn/qt0kf9t6gn.pdf?t=oruwia | doi-access = free }}</ref><ref>{{cite journal | vauthors = Bäckström D, Yutin N, Jørgensen SL, Dharamshi J, Homa F, Zaremba-Niedwiedzka K, Spang A, Wolf YI, Koonin EV, Ettema TJ | title = Virus Genomes from Deep Sea Sediments Expand the Ocean Megavirome and Support Independent Origins of Viral Gigantism | journal = mBio | volume = 10 | issue = 2 |pages=e02497-02418 | date = March 2019 | doi = 10.1128/mBio.02497-18 | pmid = 30837339 | pmc = 6401483 }}</ref><ref>{{cite journal | vauthors = Yutin N, Wolf Y, Koonin EV | title = Origin of giant viruses from smaller DNA viruses not from a fourth domain of cellular life | journal = Virology | volume = 466–467 | issue = 2014 |pages=38-52 | date = 2014 | doi = 10.1016/j.virol.2014.06.032 |pmc=4325995 }}</ref> |
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==Comparison of largest known giant viruses== |
==Comparison of largest known giant viruses== |
Megaviricetes | |
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![]() | |
Mimivirus | |
Virus classification ![]() | |
(unranked): | Virus |
Realm: | Varidnaviria |
Kingdom: | Bamfordvirae |
Phylum: | Nucleocytoviricota |
Class: | Megaviricetes |
Agiant virus, sometimes referred to as a girus, is a very large virus, some of which are larger than typical bacteria.[1][2] All known giant viruses belong to the phylum Nucleocytoviricota.[3]
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)[4] 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).[3][5] While few giant viruses have been characterized in detail, the most notable examples are the phylogenetically related mimivirus and megavirus—both belonging to the Mimiviridae (aka Megaviridae) family, due to their having the largest capsid diameters of all known viruses.[3][5]Giant viruses from the deep ocean, terrestrial sources, and human patients contain genes encoding cytochrome P450 (CYP; P450) enzymes. The origin of these P450 genes in giant viruses remains unknown but may have been acquired from an ancient host.[6]
The genomes of many giant viruses encode many unusual genes that are not found in other viruses, including genes involved in glycolysis and the TCA cycle[7], fermentation[8], and the cytoskeleton[9][10][11].
The first giant virus, then named BV-PW1, was isolated and described in 1995,[12] but was not recognized as such until its sequenced genome was released as Cafeteria roenbergensis virus (CroV) in 2010.[13] Subsequently, the Giant Virus Acanthamoeba polyphaga Mimivirus was characterized[14] (which had been mistaken as a bacterium in 1993),[15] and then sequenced.[16] The term "girus" was coined to refer to the group in 2006.[17]
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.[5] 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.[5]
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.[18] 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,[5] but this has been largely discounted.[19][20][21]
Giant virus name | Genome Length | Genes | Capsid diameter (nm) | Hair cover | Genbank # |
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Bodo saltans virus[22] | 1,385,869 | 1227 proteins (predicted) | ~300 | yes (~40 nm) | MF782455 |
Megavirus chilensis[23] | 1,259,197 | 1120 proteins (predicted) | 440 | yes (75 nm) | JN258408 |
Mamavirus[24] | 1,191,693 | 1023 proteins (predicted) | 500 | yes (120 nm) | JF801956 |
Mimivirus[16][25] | 1,181,549 | 979 proteins 39 non-coding | 500 | yes (120 nm) | NC_014649 |
M4[26] (Mimivirus "bald" variant) | 981,813 | 756 proteins (predicted) | 390 | No | JN036606 |
Tupanvirus[27] | 1,500,000 | 1276–1425 proteins | ≥450+550[28] | KY523104 MF405918[29] | |
Cafeteria roenbergensis virus[30] | 617,453 (730 kb) | 544 proteins (predicted) | 300 | No | NC_014637 |
The whole list is in the Giant Virus Toplist created by the Giant Virus Finder software.[31]
Giant virus name | Aminoacyl-tRNA synthetase | Octocoral-like 1MutS | 2Stargate[32] | Known virophage[33] | Cytoplasmic virion factory | Host |
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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) |
M4 (Mimivirus "bald" variant) | 3 (Met, Cys, Arg) | yes | yes | Resistant | yes | Acanthamoeba (Unikonta, Amoebozoa) |
Cafeteria roenbergensis virus | 1 (Ile) | yes | no | yes | yes | Phagotrophic protozoan (Heterokonta, Stramenopiles) |
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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Self-replicating organic structures
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