Giant virus
A giant virus, also known as a girus, is a very large virus, some of which are larger than typical bacteria. They have extremely large genomes compared to other viruses and contain many unique genes not found in life forms. All known giant viruses belong to the phylum Nucleocytoviricota ; they are the larger members of this group.
Description
While the exact criteria as defined in the scientific literature vary, giant viruses are generally described as viruses having large, pseudo-icosahedral capsids that may be surrounded by a thick layer of filamentous protein fibers. The viruses' large, double-stranded DNA genomes encode a large contingent of genes. 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 family, due to their having the largest capsid diameters of all known viruses.and APMV. Cryo-electron micrograph of four CroV particles. Single CroV particle with concave core depression. Single APMV particle. Scale bars in 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. 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.
Giant viruses from the deep ocean, terrestrial sources, and human patients contain genes encoding cytochrome P450 enzymes. The origin of these P450 genes in giant viruses remains unknown but may have been acquired from an ancient host.
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. 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.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. 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, but this has been largely discounted.
Comparison of largest known giant viruses
The whole list is in the Giant Virus Toplist created by the Giant Virus Finder software.| Giant virus name | Aminoacyl-tRNA synthetase | Octocoral-like 1MutS | 2Stargate | Known virophage | Cytoplasmic virion factory | Host |
| Megavirus chilensis | 7 | yes | yes | no | yes | Acanthamoeba |
| Mamavirus | 4 | yes | yes | yes | yes | Acanthamoeba |
| Mimivirus | 4 | yes | yes | yes | yes | Acanthamoeba |
1Mutator S 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.