A girus (a contraction of giant virus or gigantic virus) is a very large virus. They are giant nucleocytoplasmic large DNA viruses (NCLDVs), having unique genes not found in other life, with separate phylogenic trees for those genes.
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). 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.
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. 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.
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. 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.
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.
Comparison of largest known giruses
Table 1 : Largest giant viruses with complete sequenced genomes
Table 2: Specific common features among giant viruses
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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