Virus classification

Last updated

Virus classification is the process of naming viruses and placing them into a taxonomic system similar to the classification systems used for cellular organisms.

Contents

Viruses are classified by phenotypic characteristics, such as morphology, nucleic acid type, mode of replication, host organisms, and the type of disease they cause. The formal taxonomic classification of viruses is the responsibility of the International Committee on Taxonomy of Viruses (ICTV) system, although the Baltimore classification system can be used to place viruses into one of seven groups based on their manner of mRNA synthesis. Specific naming conventions and further classification guidelines are set out by the ICTV.

A catalogue of all the world's known viruses has been proposed and, in 2013, some preliminary efforts were underway. [1]

Definitions

Species definition

Species form the basis for any biological classification system. Before 1982, it was thought that viruses could not be made to fit Ernst Mayr's reproductive concept of species, and so were not amenable to such treatment. In 1982, the ICTV started to define a species as "a cluster of strains" with unique identifying qualities. In 1991, the more specific principle that a virus species is a polythetic class of viruses that constitutes a replicating lineage and occupies a particular ecological niche was adopted. [2]

In July 2013, the ICTV definition of species changed to state: "A species is a monophyletic group of viruses whose properties can be distinguished from those of other species by multiple criteria." [3] These criteria include the structure of the capsid, the existence of an envelope, the gene expression program for its proteins, host range, pathogenicity, and most importantly genetic sequence similarity and phylogenetic relationship. [4]

The actual criteria used vary by the taxon, and can be inconsistent (arbitrary similarity thresholds) or unrelated to lineage (geography) at times. [5] The matter is, for many, not yet settled. [2]

Virus definition

The currently accepted and formal definition of a 'virus' was accepted by the ICTV Executive Committee in November 2020 and ratified in March 2021, and is as follows: [6]

Viruses sensu stricto are defined operationally by the ICTV as a type of MGE (mobile genetic element) that encodes at least one protein that is a major component of the virion encasing the nucleic acid of the respective MGE and therefore the gene encoding the major virion protein itself or MGEs that are clearly demonstrable to be members of a line of evolutionary descent of such major virion protein-encoding entities. Any monophyletic group of MGEs that originates from a virion protein-encoding ancestor should be classified as a group of viruses.

ICTV classification

Comparison 1991 and 2018b virus taxonomy by ICTV Comparison 1991 and 2019 virus taxonomy ICTV.webp
Comparison 1991 and 2018b virus taxonomy by ICTV

The International Committee on Taxonomy of Viruses began to devise and implement rules for the naming and classification of viruses early in the 1970s, an effort that continues to the present. The ICTV is the only body charged by the International Union of Microbiological Societies with the task of developing, refining, and maintaining a universal virus taxonomy. [7] The system shares many features with the classification system of cellular organisms, such as taxon structure. However, some differences exist, such as the universal use of italics for all taxonomic names, unlike in the International Code of Nomenclature for algae, fungi, and plants and International Code of Zoological Nomenclature. [8]

Viral classification starts at the level of realm and continues as follows, with the taxonomic suffixes in parentheses: [8]

Realm (-viria)
Subrealm (-vira)
Kingdom (-virae)
Subkingdom (-virites)
Phylum (-viricota)
Subphylum (-viricotina)
Class (-viricetes)
Subclass (-viricetidae)
Order (-virales)
Suborder (-virineae)
Family (-viridae)
Subfamily (-virinae)
Genus (-virus)
Subgenus (-virus)
Species

Unlike the system of binomial nomenclature adopted in cellular species, there is currently no standardized form for virus species names. At present, the ICTV mandates that a species name must contain as few words as possible while remaining distinct, and must not only contain the word virus and the host name. [9] Species names often take the form of [Disease] virus, particularly for higher plants and animals. In 2019, the ICTV published a proposal to adopt a more formalized system of binomial nomenclature for virus species names, to be voted on in 2020. [10] However, some virologists later objected to the potential naming system change, arguing that the debate came while many in the field were preoccupied due to the COVID-19 pandemic. [11]

As of 2019, all levels of taxa except subrealm, subkingdom, and subclass are used. Four realms, one incertae sedis order, 24 incertae sedis families, and three incertae sedis genera are recognized: [12]

Realms: Duplodnaviria , Monodnaviria , Adnaviria , Ribozyviria , Riboviria , and Varidnaviria

Incertae sedis families and orders:

Incertae sedis genera: Dinodnavirus , Rhizidiovirus

Structure-based virus classification

It has been suggested that similarity in virion assembly and structure observed for certain viral groups infecting hosts from different domains of life (e.g., bacterial tectiviruses and eukaryotic adenoviruses or prokaryotic Caudovirales and eukaryotic herpesviruses) reflects an evolutionary relationship between these viruses. [13] Therefore, structural relationship between viruses has been suggested to be used as a basis for defining higher-level taxa – structure-based viral lineages – that could complement the ICTV classification scheme of 2010. [14]

The ICTV has gradually added many higher-level taxa using relationships in protein folds. All four realms defined in the 2019 release are defined by the presence of a protein of a certain structural family. [15]

Baltimore classification

The Baltimore Classification of viruses is based on the method of viral mRNA synthesis Baltimore Classification.svg
The Baltimore Classification of viruses is based on the method of viral mRNA synthesis

Baltimore classification (first defined in 1971) is a classification system that places viruses into one of seven groups depending on a combination of their nucleic acid (DNA or RNA), strandedness (single-stranded or double-stranded), sense, and method of replication. Named after David Baltimore, a Nobel Prize-winning biologist, these groups are designated by Roman numerals. Other classifications are determined by the disease caused by the virus or its morphology, neither of which are satisfactory due to different viruses either causing the same disease or looking very similar. In addition, viral structures are often difficult to determine under the microscope. Classifying viruses according to their genome means that those in a given category will all behave in a similar fashion, offering some indication of how to proceed with further research. Viruses can be placed in one of the seven following groups: [16]

Visualization of the 7 groups of virus according to the Baltimore Classification The Baltimore Classification.gif
Visualization of the 7 groups of virus according to the Baltimore Classification

DNA viruses

Viruses with a DNA genome, except for the DNA reverse transcribing viruses, are members of three of the four recognized viral realms: Duplodnaviria , Monodnaviria , and Varidnaviria . But the incertae sedis order Ligamenvirales , and many other incertae sedis families and genera, are also used to classify DNA viruses. The domains Duplodnaviria and Varidnaviria consist of double-stranded DNA viruses; other double-stranded DNA viruses are incertae sedis. The domain Monodnaviria consists of single-stranded DNA viruses that generally encode a HUH endonuclease; other single-stranded DNA viruses are incertae sedis. [12]

  • Group I: viruses possess double-stranded DNA. Viruses that cause chickenpox and herpes are found here.
  • Group II: viruses possess single-stranded DNA.
Examples of DNA viruses
Virus familyExamples (common names)Virion
naked/enveloped
Capsid
symmetry
Nucleic acid typeGroup
1. Adenoviridae Canine hepatitis virus, Some types of the common cold NakedIcosahedraldsI
2. Papovaviridae JC virus, HPV NakedIcosahedralds circularI
3. Parvoviridae Human parvovirus B19, canine parvovirus NakedIcosahedralssII
4. Herpesviridae Herpes simplex virus, varicella-zoster virus, cytomegalovirus, Epstein–Barr virus Enveloped IcosahedraldsI
5. Poxviridae Smallpox virus, cowpox, myxoma virus, monkeypox, vaccinia virus Complex coatsComplexdsI
6. Anelloviridae Torque teno virusNakedIcosahedralss circularII
7. Pleolipoviridae HHPV1, HRPV1 Envelopedss/ds linear/circularI/II

RNA viruses

All viruses that have an RNA genome, and that encode an RNA-dependent RNA polymerase (RdRp), are members of the kingdom Orthornavirae , within the realm Riboviria . [17]

  • Group III: viruses possess double-stranded RNA genomes, e.g. rotavirus.
  • Group IV: viruses possess positive-sense single-stranded RNA genomes. Many well known viruses are found in this group, including the picornaviruses (which is a family of viruses that includes well-known viruses like Hepatitis A virus, enteroviruses, rhinoviruses, poliovirus, and foot-and-mouth virus), SARS virus, hepatitis C virus, yellow fever virus, and rubella virus.
  • Group V: viruses possess negative-sense single-stranded RNA genomes. Ebola and Marburg viruses are well known members of this group, along with influenza virus, measles, mumps and rabies.
Examples of RNA viruses
Virus FamilyExamples (common names)Capsid
naked/enveloped
Capsid
Symmetry
Nucleic acid typeGroup
1. Reoviridae Reovirus, rotavirus NakedIcosahedraldsIII
2. Picornaviridae Enterovirus, rhinovirus, hepatovirus, cardiovirus, aphthovirus, poliovirus, parechovirus, erbovirus, kobuvirus, teschovirus, coxsackie NakedIcosahedralssIV
3. Caliciviridae Norwalk virus NakedIcosahedralssIV
4. Togaviridae Eastern equine encephalitis Enveloped IcosahedralssIV
5. Arenaviridae Lymphocytic choriomeningitis virus, Lassa fever Enveloped Complexss(−)V
6. Flaviviridae Dengue virus, hepatitis C virus, yellow fever virus, Zika virus Enveloped IcosahedralssIV
7. Orthomyxoviridae Influenzavirus A, influenzavirus B, influenzavirus C, isavirus, thogotovirus Enveloped Helicalss(−)V
8. Paramyxoviridae Measles virus, mumps virus, respiratory syncytial virus, Rinderpest virus, canine distemper virus Enveloped Helicalss(−)V
9. Bunyaviridae California encephalitis virus, Sin nombre virus Enveloped Helicalss(−)V
10. Rhabdoviridae Rabies virus, Vesicular stomatitis Enveloped Helicalss(−)V
11. Filoviridae Ebola virus, Marburg virus Enveloped Helicalss(−)V
12. Coronaviridae Human coronavirus 229E, Human coronavirus NL63, Human coronavirus OC43, Human coronavirus HKU1, Middle East respiratory syndrome-related coronavirus, Severe acute respiratory syndrome coronavirus, and Severe acute respiratory syndrome coronavirus 2 Enveloped HelicalssIV
13. Astroviridae Astrovirus NakedIcosahedralssIV
14. Bornaviridae Borna disease virus Enveloped Helicalss(−)V
15. Arteriviridae Arterivirus, equine arteritis virus Enveloped IcosahedralssIV
16. Hepeviridae Hepatitis E virus NakedIcosahedralssIV

Reverse transcribing viruses

All viruses that encode a reverse transcriptase (also known as RT or RNA-dependent DNA polymerase) are members of the class Revtraviricetes , within the phylum Arterviricota, kingdom Pararnavirae, and realm Riboviria . The class Blubervirales contains the single family Hepadnaviridae of DNA RT (reverse transcribing) viruses; all other RT viruses are members of the class Ortervirales. [18]

  • Group VI: viruses possess single-stranded RNA viruses that replicate through a DNA intermediate. The retroviruses are included in this group, of which HIV is a member.
  • Group VII: viruses possess double-stranded DNA genomes and replicate using reverse transcriptase. The hepatitis B virus can be found in this group.
Examples of reverse transcribing viruses
Virus FamilyExamples (common names)Capsid
naked/enveloped
Capsid
Symmetry
Nucleic acid typeGroup
1. Retroviridae HIV Enveloped dimer RNAVI
2. Caulimoviridae Caulimovirus, Cacao swollen-shoot virus (CSSV)NakedVII
3. Hepadnaviridae Hepatitis B virus Enveloped Icosahedralcircular, partially dsVII

Historical systems

Holmes classification

Holmes (1948) used a Linnaean taxonomy with binomial nomenclature to classify viruses into 3 groups under one order, Virales. They are placed as follows:

The system was not accepted by others due to its neglect of morphological similarities. [19]

Subviral agents

The following infectious agents are smaller than viruses and have only some of their properties. [20] [21] Since 2015, the ICTV has allowed them to be classified in a similar way as viruses are. [22]

Viroids and virus-dependent agents

Viroids

Satellites

Satellites depend on co-infection of a host cell with a helper virus for productive multiplication. Their nucleic acids have substantially distinct nucleotide sequences from either their helper virus or host. When a satellite subviral agent encodes the coat protein in which it is encapsulated, it is then called a satellite virus.

Satellite-like nucleic acids resemble satellite nucleic acids, in that they replicate with the aid of helper viruses. However they differ in that they can encode functions that can contribute to the success of their helper viruses; while they are sometimes considered to be genomic elements of their helper viruses, they are not always found within their helper viruses. [20]

Defective interfering particles

Defective interfering particles are defective viruses that have lost their ability to replicate except in the presence of a helper virus, which is normally the parental virus. They can also interfere with the helper virus.

  • Defective interfering particles (RNA)
  • Defective interfering particles (DNA)

See also

Notes

  1. Zimmer C (5 September 2013). "A Catalog for All the World's Viruses?". New York Times . Retrieved 6 September 2013.
  2. 1 2 Alimpiev, Egor (March 15, 2019). Rethinking the Virus Species Concept (PDF).
  3. Adams MJ, Lefkowitz EJ, King AM, Carstens EB (December 2013). "Recently agreed changes to the International Code of Virus Classification and Nomenclature". Archives of Virology. 158 (12): 2633–9. doi: 10.1007/s00705-013-1749-9 . PMID   23836393.
  4. "International Committee on Taxonomy of Viruses (ICTV)". International Committee on Taxonomy of Viruses (ICTV). Retrieved 2021-06-10.
  5. Peterson, A Townsend (23 July 2014). "Defining viral species: making taxonomy useful". Virology Journal. 11 (1): 131. doi:10.1186/1743-422X-11-131. PMC   4222810 . PMID   25055940.
  6. Koonin, Eugene V.; Dolja, Valerian V.; Krupovic, Mart; Kuhn, Jens H. (2021-09-01). "Viruses Defined by the Position of the Virosphere within the Replicator Space". Microbiology and Molecular Biology Reviews. 85 (4): e0019320. doi:10.1128/MMBR.00193-20. PMC   8483706 . PMID   34468181.
  7. Lefkowitz EJ, Dempsey DM, Hendrickson RC, Orton RJ, Siddell SG, Smith DB (January 2018). "Virus taxonomy: the database of the International Committee on Taxonomy of Viruses (ICTV)". Nucleic Acids Research. 46 (D1): D708–D717. doi:10.1093/nar/gkx932. PMC   5753373 . PMID   29040670.
  8. 1 2 "ICTV Code". talk.ictvonline.org. International Committee on Taxonomy of Viruses. Retrieved 26 April 2020.
  9. "The International Code of Virus Classification and Nomenclature". ICTV. International Committee on Taxonomy of Viruses. Retrieved 2 September 2020.
  10. Siddell, Stuart; Walker, Peter; Lefkowitz, Elliot; Mushegian, Arcady; Dutilh, Bas; Harrach, Balázs; Harrison, Robert; Junglen, Sandra; Knowles, Nick; Kropinski, Andrew; Krupovic, Mart; Kuhn, Jens; Nibert, Max; Rubino, Luisa; Sabanadzovic, Sead; Simmonds, Peter; Varsani, Arvind; Zerbini, Francisco; Davison, Andrew (3 December 2019). "Binomial nomenclature for virus species: a consultation". Arch Virol. 165 (2): 519–525. doi: 10.1007/s00705-019-04477-6 . PMC   7026202 . PMID   31797129.
  11. Mallapaty, Smriti (30 July 2020). "Should virus-naming rules change during a pandemic? The question divides virologists". Nature. 584 (7819): 19–20. Bibcode:2020Natur.584...19M. doi: 10.1038/d41586-020-02243-2 . PMID   32733098.
  12. 1 2 "Virus Taxonomy: 2019 Release". talk.ictvonline.org. International Committee on Taxonomy of Viruses. Retrieved 26 April 2020.
  13. Bamford DH (May 2003). "Do viruses form lineages across different domains of life?". Research in Microbiology. 154 (4): 231–6. doi:10.1016/S0923-2508(03)00065-2. PMID   12798226.
  14. Krupovič M, Bamford DH (December 2010). "Order to the viral universe". Journal of Virology. 84 (24): 12476–9. doi:10.1128/JVI.01489-10. PMC   3004316 . PMID   20926569.
  15. "Virus Taxonomy: 2019 Release". talk.ictvonline.org. International Committee on Taxonomy of Viruses. Retrieved 26 April 2020.
  16. "Baltimore Classification of Viruses" (Website.) Molecular Biology Web Book – http://web-books.com/ . Retrieved on 2008-08-18.
  17. "Virus Taxonomy: 2019 Release". talk.ictvonline.org. International Committee on Taxonomy of Viruses. Retrieved 25 April 2020.
  18. Koonin EV, Dolja VV, Krupovic M, Varsani A, Wolf YI, Yutin N, Zerbini M, Kuhn JH. "Proposal: Create a megataxonomic framework, filling all principal taxonomic ranks, for realm Riboviria". International Committee on Taxonomy of Viruses (ICTV). Retrieved 2020-05-21.{{cite web}}: CS1 maint: multiple names: authors list (link)
  19. Kuhn, Jens H. (2020). "Virus Taxonomy". Reference Module in Life Sciences. Encyclopedia of Virology. pp. 28–37. doi:10.1016/B978-0-12-809633-8.21231-4. ISBN   978-0-12-809633-8. PMC   7157452 .
  20. 1 2 "ICTV 9th Report (2011) Subviral Agents". International Committee on Taxonomy of Viruses. Retrieved 2020-06-15. (updated version in sync with current release)
  21. STRAUSS, JAMES H.; STRAUSS, ELLEN G. (2008). "Subviral Agents". Viruses and Human Disease. Elsevier. pp. 345–368. doi:10.1016/b978-0-12-373741-0.50012-x. ISBN   978-0-12-373741-0. S2CID   80872659.
  22. TaxoProp 2015.002aG
  23. "80.002 Avsunviroidae – ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2007-09-27.
  24. "80.001 Popsiviroidae – ICTVdB Index of Viruses." (Website.) U.S. National Institutes of Health website. Retrieved on 2007-09-27.
  25. Krupovic, Mart; Kuhn, Jens H.; Fischer, Matthias G. (7 October 2015). "A classification system for virophages and satellite viruses". Archives of Virology. 161 (1): 233–247. doi: 10.1007/s00705-015-2622-9 . PMID   26446887.

Related Research Articles

<span class="mw-page-title-main">DNA virus</span> Virus that has DNA as its genetic material

A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and those that have one strand of DNA in their genome, called single-stranded DNA (ssDNA) viruses. dsDNA viruses primarily belong to two realms: Duplodnaviria and Varidnaviria, and ssDNA viruses are almost exclusively assigned to the realm Monodnaviria, which also includes some dsDNA viruses. Additionally, many DNA viruses are unassigned to higher taxa. Reverse transcribing viruses, which have a DNA genome that is replicated through an RNA intermediate by a reverse transcriptase, are classified into the kingdom Pararnavirae in the realm Riboviria.

<span class="mw-page-title-main">Retrovirus</span> Family of viruses

A retrovirus is a type of virus that inserts a DNA copy of its RNA genome into the DNA of a host cell that it invades, thus changing the genome of that cell. Once inside the host cell's cytoplasm, the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus retro (backwards). The new DNA is then incorporated into the host cell genome by an integrase enzyme, at which point the retroviral DNA is referred to as a provirus. The host cell then treats the viral DNA as part of its own genome, transcribing and translating the viral genes along with the cell's own genes, producing the proteins required to assemble new copies of the virus.

<span class="mw-page-title-main">RNA virus</span> Subclass of viruses

An RNA virus is a virus—other than a retrovirus—that has ribonucleic acid (RNA) as its genetic material. The nucleic acid is usually single-stranded RNA (ssRNA) but it may be double-stranded (dsRNA). Notable human diseases caused by RNA viruses include the common cold, influenza, SARS, MERS, Covid-19, Dengue Virus, hepatitis C, hepatitis E, West Nile fever, Ebola virus disease, rabies, polio, mumps, and measles.

Viroids are small single-stranded, circular RNAs that are infectious pathogens. Unlike viruses, they have no protein coating. All known viroids are inhabitants of angiosperms, and most cause diseases, whose respective economic importance to humans varies widely.

Virusoids are circular single-stranded RNA(s) dependent on viruses for replication and encapsidation. The genome of virusoids consist of several hundred (200–400) nucleotides and does not code for any proteins.

A satellite is a subviral agent that depends on the coinfection of a host cell with a helper virus for its replication. Satellites can be divided into two major classes: satellite viruses and satellite nucleic acids. Satellite viruses, which are most commonly associated with plants, are also found in mammals, arthropods, and bacteria. They encode structural proteins to enclose their genetic material, which are therefore distinct from the structural proteins of their helper viruses. Satellite nucleic acids, in contrast, do not encode their own structural proteins, but instead are encapsulated by proteins encoded by their helper viruses. The genomes of satellites range upward from 359 nucleotides in length for satellite tobacco ringspot virus RNA (STobRV).

<i>Geminiviridae</i> Family of viruses

Geminiviridae is a family of plant viruses that encode their genetic information on a circular genome of single-stranded (ss) DNA. There are 520 species in this family, assigned to 14 genera. Diseases associated with this family include: bright yellow mosaic, yellow mosaic, yellow mottle, leaf curling, stunting, streaks, reduced yields. They have single-stranded circular DNA genomes encoding genes that diverge in both directions from a virion strand origin of replication. According to the Baltimore classification they are considered class II viruses. It is the largest known family of single stranded DNA viruses.

Pseudoviridae is a family of viruses, which includes three genera.

<span class="mw-page-title-main">International Committee on Taxonomy of Viruses</span> International organisation that regulates classification and nomenclature of viruses

The International Committee on Taxonomy of Viruses (ICTV) authorizes and organizes the taxonomic classification of and the nomenclatures for viruses. The ICTV has developed a universal taxonomic scheme for viruses, and thus has the means to appropriately describe, name, and classify every virus that affects living organisms. The members of the International Committee on Taxonomy of Viruses are considered expert virologists. The ICTV was formed from and is governed by the Virology Division of the International Union of Microbiological Societies. Detailed work, such as delimiting the boundaries of species within a family, typically is performed by study groups of experts in the families.

<span class="mw-page-title-main">Baltimore classification</span> Virus classification system made by David Baltimore

Baltimore classification is a system used to classify viruses based on their manner of messenger RNA (mRNA) synthesis. By organizing viruses based on their manner of mRNA production, it is possible to study viruses that behave similarly as a distinct group. Seven Baltimore groups are described that take into consideration whether the viral genome is made of deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), whether the genome is single- or double-stranded, and whether the sense of a single-stranded RNA genome is positive or negative.

<span class="mw-page-title-main">Double-stranded RNA viruses</span> Type of virus according to Baltimore classification

Double-stranded RNA viruses are a polyphyletic group of viruses that have double-stranded genomes made of ribonucleic acid. The double-stranded genome is used to transcribe a positive-strand RNA by the viral RNA-dependent RNA polymerase (RdRp). The positive-strand RNA may be used as messenger RNA (mRNA) which can be translated into viral proteins by the host cell's ribosomes. The positive-strand RNA can also be replicated by the RdRp to create a new double-stranded viral genome.

Yingchengvirus is a genus of double stranded DNA viruses that infect haloarchaea. The genus was previously named Betasphaerolipovirus.

<span class="mw-page-title-main">Positive-strand RNA virus</span> Class of viruses in the Baltimore classification

Positive-strand RNA viruses are a group of related viruses that have positive-sense, single-stranded genomes made of ribonucleic acid. The positive-sense genome can act as messenger RNA (mRNA) and can be directly translated into viral proteins by the host cell's ribosomes. Positive-strand RNA viruses encode an RNA-dependent RNA polymerase (RdRp) which is used during replication of the genome to synthesize a negative-sense antigenome that is then used as a template to create a new positive-sense viral genome.

<i>Genomoviridae</i> Family of viruses

Genomoviridae is a family of single stranded DNA viruses that mainly infect fungi. The genomes of this family are small. The genomes are circular single-stranded DNA and encode rolling-circle replication initiation proteins (Rep) and unique capsid proteins. In Rep-based phylogenies, genomoviruses form a sister clade to plant viruses of the family Geminiviridae. Ten genera are recognized in this family.

<i>Riboviria</i> Realm of viruses

Riboviria is a realm of viruses that includes all viruses that use a homologous RNA-dependent polymerase for replication. It includes RNA viruses that encode an RNA-dependent RNA polymerase, as well as reverse-transcribing viruses that encode an RNA-dependent DNA polymerase. RNA-dependent RNA polymerase (RdRp), also called RNA replicase, produces RNA from RNA. RNA-dependent DNA polymerase (RdDp), also called reverse transcriptase (RT), produces DNA from RNA. These enzymes are essential for replicating the viral genome and transcribing viral genes into messenger RNA (mRNA) for translation of viral proteins.

In virology, realm is the highest taxonomic rank established for viruses by the International Committee on Taxonomy of Viruses (ICTV), which oversees virus taxonomy. Six virus realms are recognized and united by specific highly conserved traits:

<i>Varidnaviria</i> Realm of viruses

Varidnaviria is a realm of viruses that includes all DNA viruses that encode major capsid proteins that contain a vertical jelly roll fold. The major capsid proteins (MCP) form into pseudohexameric subunits of the viral capsid, which stores the viral deoxyribonucleic acid (DNA), and are perpendicular, or vertical, to the surface of the capsid. Apart from this, viruses in the realm also share many other characteristics, such as minor capsid proteins (mCP) with the vertical jelly roll fold, an ATPase that packages viral DNA into the capsid, and a DNA polymerase that replicates the viral genome.

<i>Albetovirus</i> Genus of virus

Albetovirus is a plant satellite virus genus. As a member of realm Riboviria without assigned family or order it contains just three species, Tobacco albetovirus 1, 2, and 3. The three virus species included in this genus represent satellite viruses that depend for their replication on helper viruses of the genera Alphanecrovirus or Betanecrovirus

<i>Ribozyviria</i> Realm of viruses

Ribozyviria is a realm of satellite nucleic acids. Established in ICTV TaxoProp 2020.012D, the realm is named after the presence of genomic and antigenomic ribozymes of the Deltavirus type. Additional common features include a rod-like structure, a RNA-binding "delta antigen" encoded in the genome, and animal hosts. Furthermore, the size range of the genomes of these viruses is between around 1547–1735nt, they encode a hammerhead ribozyme or a hepatitis delta virus ribozyme, and their coding capacity only involves one conserved protein. Most lineages of this realm are poorly understood, the notable exception being members of the genus Deltavirus, the causal agents of Hepatitis D in humans.

<i>Adnaviria</i> Realm of viruses

Adnaviria is a realm of viruses that includes archaeal viruses that have a filamentous virion and a linear, double-stranded DNA genome. The genome exists in A-form (A-DNA) and encodes a dimeric major capsid protein (MCP) that contains the SIRV2 fold, a type of alpha-helix bundle containing four helices. The virion consists of the genome encased in capsid proteins to form a helical nucleoprotein complex. For some viruses, this helix is surrounded by a lipid membrane called an envelope. Some contain an additional protein layer between the nucleoprotein helix and the envelope. Complete virions are long and thin and may be flexible or a stiff like a rod.