Giant virus

Giant virus
Virus classification
Group: Group I (dsDNA)

A giant virus is a very large virus, some of which are larger than typical bacteria.[1][2] 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.[3]

Description

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).[3][4] 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.[3][4]

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.

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.[4] 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.[4] 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.[4]

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.[5] 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,[4] but this is not universally accepted.[6]

Comparison of largest known giant viruses

Table 1 - Largest giant viruses with complete sequenced genomes
Giant virus nameGenome LengthGenesCapsid diameter (nm)Hair coverGenbank #
Megavirus chilensis[7]1,259,1971120 proteins (predicted)440yes (75 nm)JN258408
Mamavirus[8]1,191,6931023 proteins (predicted)390yes (120 nm)JF801956
Mimivirus[9][10]1,181,549979 proteins 39 non-coding390yes (120 nm)NC_014649
Tupanvirus[11]1,500,0001276-1425 proteins≥450+550[12]KY523104
MF405918[13]

The whole list is in the Giant Virus Toplist created by the Giant Virus Finder software.[14]

Table 2 - Specific common features among giant viruses
Giant virus nameAminoacyl-tRNA synthetaseOctocoral-like 1MutS2Stargate[15]Known virophage[16]Cytoplasmic virion factoryHost
Megavirus chilensis7 (Tyr, Arg, Met, Cys, Trp, Asn, Ile)yesyesnoyesAcanthamoeba (Unikonta, Amoebozoa)
Mamavirus4 (Tyr, Arg, Met, Cys)yesyesyesyesAcanthamoeba (Unikonta, Amoebozoa)
Mimivirus4 (Tyr, Arg, Met, Cys)yesyesyesyesAcanthamoeba (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.

See also

References

  1. ↑ Reynolds, Kelly A. (2010). "Mysterious Microbe in Water Challenges the Very Definition of a Virus" (PDF). Water Conditioning & Purification. Archived from the original (PDF) on 2014-03-19.
  2. ↑ Ogata, Hiroyuki; Kensuke Toyoda; Yuji Tomaru; Natsuko Nakayama; Yoko Shirai; Jean-Michel Claverie; Keizo Nagasaki (2009). "Remarkable sequence similarity between the dinoflagellate-infecting marine girus and the terrestrial pathogen African swine fever virus". Virology Journal. 6 (178): 178. doi:10.1186/1743-422X-6-178. PMC 2777158. PMID 19860921. Retrieved 30 May 2011.
  3. 1 2 3 Van Etten, James L. (July–August 2011). "Giant Viruses". American Scientist. 99 (4): 304–311. doi:10.1511/2011.91.304.
  4. 1 2 3 4 5 6 Legendre, Matthieu; Defne Arslan; Chantal Abergel; Jean-Michel Claverie (2012). "Genomics of Megavirus and the elusive fourth domain of Life". Communicative & Integrative Biology. 5 (1): 102–6. doi:10.4161/cib.18624. PMC 3291303. PMID 22482024.
  5. ↑ [In Giant Virus Genes, Hints About Their Mysterious Origin https://www.npr.org/sections/health-shots/2017/04/06/522478901/in-giant-virus-genes-hints-about-their-mysterious-origin]
  6. ↑ Schulz, Frederik; Yutin, Natalya; Ivanova, Natalia N.; Ortega, Davi R.; Lee, Tae Kwon; Vierheilig, Julia; Daims, Holger; Horn, Matthias; Wagner, Michael (2017-04-07). "Giant viruses with an expanded complement of translation system components". Science. 356 (6333): 82–85. doi:10.1126/science.aal4657. ISSN 0036-8075. PMID 28386012.
  7. ↑ Arslan, D.; Legendre, M.; Seltzer, V.; Abergel, C.; Claverie, J.-M. (2011). "Distant Mimivirus relative with a larger genome highlights the fundamental features of Megaviridae". Proceedings of the National Academy of Sciences. 108 (42): 17486–91. doi:10.1073/pnas.1110889108. PMC 3198346. PMID 21987820.
  8. ↑ Colson, P.; Yutin, N.; Shabalina, S. A.; Robert, C.; Fournous, G.; La Scola, B.; Raoult, D.; Koonin, E. V. (2011). "Viruses with More Than 1,000 Genes: Mamavirus, a New Acanthamoeba polyphaga mimivirus Strain, and Reannotation of Mimivirus Genes". Genome Biology and Evolution. 3: 737–42. doi:10.1093/gbe/evr048. PMC 3163472. PMID 21705471.
  9. ↑ Raoult, D.; Audic, S; Robert, C; Abergel, C; Renesto, P; Ogata, H; La Scola, B; Suzan, M; Claverie, JM (2004). "The 1.2-Megabase Genome Sequence of Mimivirus". Science. 306 (5700): 1344–50. doi:10.1126/science.1101485. PMID 15486256.
  10. ↑ Legendre, Matthieu; Santini, Sébastien; Rico, Alain; Abergel, Chantal; Claverie, Jean-Michel (2011). "Breaking the 1000-gene barrier for Mimivirus using ultra-deep genome and transcriptome sequencing". Virology Journal. 8 (1): 99. doi:10.1186/1743-422X-8-99. PMC 3058096. PMID 21375749.
  11. ↑ Abrahão, Jônatas; Silva, Lorena; Silva, Ludmila Santos; Khalil, Jacques Yaacoub Bou; Rodrigues, Rodrigo; Arantes, Thalita; Assis, Felipe; Boratto, Paulo; Andrade, Miguel; Kroon, Erna Geessien; Ribeiro, Bergmann; Bergier, Ivan; Seligmann, Herve; Ghigo, Eric; Colson, Philippe; Levasseur, Anthony; Kroemer, Guido; Raoult, Didier; Scola, Bernard La (27 February 2018). "Tailed giant Tupanvirus possesses the most complete translational apparatus of the known virosphere". Nature Communications. 9 (1). doi:10.1038/s41467-018-03168-1.
  12. ↑ haed and tail, respectively
  13. ↑ soda lake and deep ocean species of Tupanvirues, respectively
  14. ↑ http://pitgroup.org/giant-virus-toplist/
  15. ↑ Zauberman, Nathan; Mutsafi, Yael; Halevy, Daniel Ben; Shimoni, Eyal; Klein, Eugenia; Xiao, Chuan; Sun, Siyang; Minsky, Abraham (2008). Sugden, Bill, ed. "Distinct DNA Exit and Packaging Portals in the Virus Acanthamoeba polyphaga mimivirus". PLoS Biology. 6 (5): e114. doi:10.1371/journal.pbio.0060114. PMC 2430901. PMID 18479185.
  16. ↑ Fischer, M. G.; Suttle, C. A. (2011). "A Virophage at the Origin of Large DNA Transposons". Science. 332 (6026): 231–4. doi:10.1126/science.1199412. PMID 21385722.
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