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Virus classification
Group: Group I (dsDNA)

A girus (a contraction of giant virus or gigantic virus) is a very large virus.[1][2] They are giant nucleocytoplasmic large DNA viruses (NCLDVs), having unique genes not found in other life, with separate phylogenic trees for those genes.[3]


While the exact criteria as defined in the scientific literature vary, giruses are generally described as those viruses having large pseudo-icosahedral capsids (200 to 400 nanometers) surrounded by a thick (approximately 100 nm) layer of filamentous protein fibers with large double-stranded DNA genomes (300 to 1000 kilobase pairs or larger) encoding a large contingent of genes (of the order of 1000 genes).[3][4] While few have been characterized in detail, the most notable examples of giruses are the phylogenetically related megavirus and mimivirus, belonging to the Megaviridae and Mimiviridae families, respectively, having the largest capsid diameters of all known viruses.[3][4]

Viral replication in giruses occurs within large circular virion factories located within the cytoplasm of the infected host cell, similar to the replication mechanism utilized by Poxviridae, though whether this mechanism is employed by all giruses 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[edit]

Girus genomes, in addition to being the largest known for viruses, 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]

The discovery and subsequent characterization of these giant viruses has triggered some debate concerning the evolutionary origins of the giruses, going so far as to suggest that the giruses provide evidence of a fourth domain of life.[4][5]

Comparison of largest known giruses[edit]

Table 1 : Largest giant viruses with complete sequenced genomes

Giant virus name Genome Length Genes Capsid diameter (nm) Hair cover Genbank #
Megavirus chilensis[6] 1,259,197 1120 proteins (predicted) 440 yes (75 nm) JN258408
Mamavirus[7] 1,191,693 1023 proteins (predicted) 390 yes (120 nm) JF801956
Mimivirus[8][9] 1,181,549 979 proteins 39 non-coding 390 yes (120 nm) NC_014649

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

Table 2: Specific common features among giant viruses

Giant virus name Aminoacyl-tRNA synthetase Octocoral-like 1MutS 2Stargate[10] Known virophage[11] 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.

See also[edit]


  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. PMC 2777158Freely accessible. PMID 19860921. doi:10.1186/1743-422X-6-178. Retrieved 30 May 2011. 
  3. ^ a b c Van Etten, James L. (July–August 2011). "Giant Viruses". American Scientist. 99 (4): 304–311. doi:10.1511/2011.91.304. 
  4. ^ a b c d e f 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. PMC 3291303Freely accessible. PMID 22482024. doi:10.4161/cib.18624. 
  5. ^ Fileé J, Chandler M. (2012) Unpacking the Baggage: Origin and Evolution of Giant Viruses. In: G.Witzany (ed). Viruses: Essential Agents of Life. Springer, 203–216. ISBN 978-94-007-4898-9.
  6. ^ 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. PMC 3198346Freely accessible. PMID 21987820. doi:10.1073/pnas.1110889108. 
  7. ^ 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. PMC 3163472Freely accessible. PMID 21705471. doi:10.1093/gbe/evr048. 
  8. ^ 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. PMID 15486256. doi:10.1126/science.1101485. 
  9. ^ 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. PMC 3058096Freely accessible. PMID 21375749. doi:10.1186/1743-422X-8-99. 
  10. ^ 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. PMC 2430901Freely accessible. PMID 18479185. doi:10.1371/journal.pbio.0060114. 
  11. ^ Fischer, M. G.; Suttle, C. A. (2011). "A Virophage at the Origin of Large DNA Transposons". Science. 332 (6026): 231–4. PMID 21385722. doi:10.1126/science.1199412.