"Girus" redirects here; not to be confused withGyrus.
Agiant virus, sometimes referred to as agirus, is a very largevirus, some of which are larger than typical bacteria.[1][2] All known giant viruses belong to the phylumNucleocytoviricota.[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 in diameter)[4] that may be surrounded by a thick (approximately 100 nm) layer of filamentous protein fibers. The viruses have large, double-stranded DNA genomes (300 to >1000 kilobasepairs) that encode a large contingent of genes (of the order of 1000 genes).[3][5] The best characterized giant viruses are the phylogenetically relatedmimivirus andmegavirus, which belong to the familyMimiviridae (akaMegaviridae), and are distinguished by their large capsid diameters.[3][5] Giant viruses from the deep ocean, terrestrial sources, and human patients contain genes encodingcytochrome 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 inglycolysis and theTCA cycle,[7] fermentation,[8] and thecytoskeleton.[9][10][11]
Cryo-EM images of the giant virusesCroV andAPMV. (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.Phylogeny of Nucleocytoviricota[12]
The first giant viruses to be described were chloroviruses of the familyPhycodnaviridae. These were discovered in 1981 by Russel H. Meints, James L. Van Etten, Daniel Kuczmarski, Kit Lee, and Barbara Ang. The firstchlorovirus was initially called HVCV (Hydra viridis Chlorella virus) since it was first found to infect Chlorella-like algae.[13][14]
Other giant viruses that infected marine flagellates were described later. The firstmimivirus (BV-PW1) was described in 1995,[15] but was not recognized as such until its sequenced genome was released asCafeteria roenbergensis virus (CroV) in 2010.[16] Subsequently, the Giant VirusAcanthamoeba polyphaga Mimivirus was characterized[17] (which had been mistaken as a bacterium in 1993),[18] and then sequenced.[19] The term "girus" was coined to refer to the group in 2006.[20]
The genomes of giant viruses are the largest known for viruses, and contain genes that encode for important elements oftranslation machinery, a characteristic that had previously been believed to be indicative of cellular organisms. These genes include multiple genes encoding a number ofaminoacyl tRNA synthetases, enzymes that catalyze theesterification of specific amino acids or their precursors to their corresponding cognatetRNAs to form anaminoacyl tRNA that is then used during translation.[5] The presence of four aminoacyl tRNA synthetase encoding genes in mimivirus andmamavirus genomes, both species within theMimiviridae family, as well as the discovery of seven aminoacyl tRNA synthetase genes in the megavirus genome (including those inMimiviridae) provide evidence that these large DNA viruses may have evolved from a shared cellular genome ancestor by means ofgenome reduction.[5]
The discovery and subsequent characterization of giant viruses has triggered debate on their evolutionary origins. The two main hypotheses are that they evolved from small viruses by picking up DNA from host organisms; or that they evolved from very complicated organisms viagenome reduction, losing various functions including self-reproduction.[21] The possible complicated ancestral organism is also a topic of debate: by one proposal, it might represent a fourthdomain of life,[5] but this has been largely discounted.[22][23][24] A molecular dating study shows that the divergence time of the last common ancestor of the giant viruses was within the last billion years, which is much younger than the origins of the known domains of life.[25]
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.
^Meints, Russel H.; Van Etten, James L.; Kuczmarski, Daniel; Lee, Kit; Ang, Barbara (September 1981). "Viral infection of the symbiotic chlorella-like alga present in Hydra viridis".Virology.113 (2):698–703.doi:10.1016/0042-6822(81)90198-7.PMID18635088.
^La Scola B, Audic S, Robert C, Jungang L, de Lamballerie X, Drancourt M, Birtles R, Claverie JM, Raoult D (2003). "A giant virus in amoebae".Science.299 (5615): 2033.doi:10.1126/science.1081867.PMID12663918.S2CID39606235.
