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Una ilustración de las "vías" por las que pasa cada grupo de Baltimore para sintetizar el ARNm.

La clasificación de Baltimore es un sistema que se utiliza para clasificar los virus según su forma de síntesis de ARN mensajero (ARNm). Al organizar los virus en función de su forma de producción de ARNm, es posible estudiar virus que se comportan de manera similar como un grupo distinto. Se describen siete grupos de Baltimore que tienen en cuenta si el genoma viral está compuesto por ácido desoxirribonucleico (ADN) o ácido ribonucleico (ARN), si el genoma es monocatenario o bicatenario y si el sentido de un genoma de ARN monocatenario. es positivo o negativo.

La clasificación de Baltimore también se corresponde estrechamente con la forma de replicar el genoma, por lo que la clasificación de Baltimore es útil para agrupar los virus para su transcripción y replicación. Ciertos sujetos pertenecientes a virus están asociados con múltiples grupos de Baltimore específicos, tales como formas específicas de traducción de ARNm y la gama de hospedadores de diferentes tipos de virus. Las características estructurales como la forma de la cápside viral , que almacena el genoma viral, y la historia evolutiva de los virus no están necesariamente relacionadas con los grupos de Baltimore.

La clasificación de Baltimore fue creada en 1971 por el virólogo David Baltimore . Desde entonces, se ha vuelto común entre los virólogos utilizar la clasificación de Baltimore junto con la taxonomía de virus estándar, que se basa en la historia evolutiva. En 2018 y 2019, la clasificación de Baltimore se integró parcialmente en la taxonomía de virus según la evidencia de que ciertos grupos descendían de ancestros comunes. Varios reinos, reinos y filos ahora corresponden a grupos específicos de Baltimore.

Resumen [ editar ]

La clasificación de Baltimore agrupa a los virus según su forma de síntesis de ARNm. Las características directamente relacionadas con esto incluyen si el genoma está hecho de ácido desoxirribonucleico (ADN) o ácido ribonucleico (ARN), la hebra del genoma, que puede ser monocatenario o bicatenario, y el sentido de un genoma monocatenario. , que es positivo o negativo. La principal ventaja de la clasificación de Baltimore es que al clasificar los virus de acuerdo con las características antes mencionadas, los virus que se comportan de la misma manera pueden estudiarse como grupos distintos. Hay siete grupos de Baltimore numerados con números romanos, que se enumeran a continuación. [1] [2]

  • Grupo I: virus de ADN bicatenario
  • Grupo II: virus de ADN monocatenario
  • Grupo III: virus de ARN bicatenario
  • Grupo IV: virus de ARN monocatenario de sentido positivo
  • Grupo V: virus de ARN monocatenario de sentido negativo
  • Grupo VI: virus de ARN monocatenario con un intermedio de ADN en su ciclo de vida
  • Grupo VII: virus de ADN de doble hebra con un intermedio de ARN en su ciclo de vida

La clasificación de Baltimore se basa principalmente en la transcripción del genoma viral, y los virus dentro de cada grupo suelen compartir las formas en que se produce la síntesis de ARNm. Si bien no es el enfoque directo de la clasificación de Baltimore, los grupos están organizados de tal manera que los virus de cada grupo también suelen tener los mismos mecanismos de replicación del genoma viral. [3] [4] Debido a esto, la clasificación de Baltimore proporciona información sobre las partes de transcripción y replicación del ciclo de vida viral . Características estructurales de una partícula de virus, llamada virión, como la forma de la cápside viral y la presencia de una envoltura viral , un lípido membrane that surrounds the capsid, have no direct relation to Baltimore groups, nor do the groups necessarily show genetic relation based on evolutionary history.[2]

Visualization of the seven groups of virus according to the Baltimore Classification

Classification[edit]

DNA viruses[edit]

Main article: DNA virus

DNA viruses have genomes made of deoxyribonucleic acid (DNA) and are organized into two groups: double-stranded DNA (dsDNA) viruses, and single-stranded DNA (ssDNA) viruses. They are assigned to three separate realms: Duplodnaviria, Monodnaviria, and Varidnaviria. Many have yet to be assigned to one of these taxa.

Grupo I: virus de ADN bicatenario [ editar ]

Ver también: Duplodnaviria y Varidnaviria

El primer grupo de Baltimore contiene virus que tienen un genoma de ADN bicatenario (dsDNA). Todos los virus dsDNA tienen su mRNA sintetizado en un proceso de tres pasos. Primero, un complejo de preiniciación de la transcripción se une al ADN corriente arriba del sitio donde comienza la transcripción, lo que permite el reclutamiento de una ARN polimerasa del hospedador . En segundo lugar, una vez que se recluta la ARN polimerasa, utiliza la hebra negativa como plantilla para sintetizar las hebras de ARNm. En tercer lugar, la ARN polimerasa termina la transcripción al alcanzar una señal específica, como un sitio de poliadenilación . [5] [6] [7]

Los virus dsDNA utilizan varios mecanismos para replicar su genoma. La replicación bidireccional, en la que dos bifurcaciones de replicación se establecen en un sitio de origen de replicación y se mueven en direcciones opuestas entre sí, se usa ampliamente. [8] También es común un mecanismo de círculo rodante que produce hebras lineales mientras avanza en un bucle alrededor del genoma circular. [9] Algunos virus de ADN bicatenario utilizan un método de desplazamiento de hebra mediante el cual una hebra se sintetiza a partir de una hebra plantilla, y luego se sintetiza una hebra complementaria a partir de la hebra sintetizada anteriormente, formando un genoma de dsDNA. [10] Por último, algunos virus dsDNA se replican como parte de un proceso llamado transposición replicativa.mediante el cual un genoma viral en el ADN de una célula huésped se replica en otra parte del genoma del huésped. [11]

Los virus dsDNA pueden subdividirse entre los que se replican en el núcleo y, como tales, son relativamente dependientes de la maquinaria de la célula huésped para la transcripción y replicación, y los que se replican en el citoplasma, en cuyo caso han evolucionado o adquirido sus propios medios para ejecutar la transcripción y replicación. [4] Los virus dsDNA también se dividen comúnmente entre virus dsDNA con cola, refiriéndose a miembros del reino Duplodnaviria , generalmente los bacteriófagos con cola del orden Caudovirales , y virus dsDNA sin cola o sin cola del reino Varidnaviria . [12] [13]

Los virus dsDNA se clasifican en tres de los cuatro reinos e incluyen muchos taxones que no están asignados a un reino:

  • Todos los virus de Duplodnaviria son virus dsDNA. Los virus en este reino pertenecen a dos grupos: bacteriófagos de cola en Caudovirales y herpesvirus en Herpesvirales . [12]
  • En Monodnaviria , los miembros de la clase Papovaviricetes son virus dsDNA. Los virus en Papovaviricetes constituyen dos grupos: papilomavirus y poliomavirus . [14]
  • Todos los virus de Varidnaviria son virus dsDNA. Los virus en este ámbito incluyen adenovirus , virus gigantes y poxvirus . [13]
  • Los siguientes taxones que no están asignados a un reino contienen exclusivamente virus dsDNA: [13]
    • Órdenes: Ligamenvirales
    • Families: Ampullaviridae, Baculoviridae, Bicaudaviridae, Clavaviridae, Fuselloviridae, Globuloviridae, Guttaviridae, Halspiviridae, Hytrosaviridae, Nimaviridae, Nudiviridae, Ovaliviridae, Plasmaviridae, Polydnaviridae, Portogloboviridae, Thaspiviridae, Tristromaviridae
    • Genera: Dinodnavirus, Rhizidiovirus

Group II: single-stranded DNA viruses[edit]

El parvovirus canino es un virus ssDNA.
Ver también: Monodnaviria

El segundo grupo de Baltimore contiene virus que tienen un genoma de ADN monocatenario (ssDNA). Los virus ssDNA tienen la misma forma de transcripción que los virus dsDNA. Sin embargo, debido a que el genoma es monocatenario, primero una ADN polimerasa lo convierte en una forma bicatenaria al entrar en una célula huésped. Luego, el ARNm se sintetiza a partir de la forma bicatenaria. La forma bicatenaria de los virus ssDNA puede producirse directamente después de la entrada en una célula o como consecuencia de la replicación del genoma viral. [15] [16] Los virus ssDNA eucariotas se replican en el núcleo. [4] [17]

Most ssDNA viruses contain circular genomes that are replicated via rolling circle replication (RCR). ssDNA RCR is initiated by an endonuclease that bonds to and cleaves the positive strand, allowing a DNA polymerase to use the negative strand as a template for replication. Replication progresses in a loop around the genome by means of extending the 3'-end of the positive strand, displacing the prior positive strand, and the endonuclease cleaves the positive strand again to create a standalone genome that is ligated into a circular loop. The new ssDNA may be packaged into virions or replicated by a DNA polymerase to form a double-stranded form for transcription or continuation of the replication cycle.[15][18]

Parvoviruses contain linear ssDNA genomes that are replicated via rolling hairpin replication (RHR), which is similar to RCR. Parvovirus genomes have hairpin loops at each end of the genome that repeatedly unfold and refold during replication to change the direction of DNA synthesis to move back and forth along the genome, producing numerous copies of the genome in a continuous process. Individual genomes are then excised from this molecule by the viral endonuclease. For parvoviruses, either the positive or negative sense strand may be packaged into capsids, varying from virus to virus.[18][19]

Nearly all ssDNA viruses have positive sense genomes, but a few exceptions and peculiarities exist. The family Anelloviridae is the only ssDNA family whose members have negative sense genomes, which are circular.[17] Parvoviruses, as previously mentioned, may package either the positive or negative sense strand into virions.[16] Lastly, bidnaviruses package both the positive and negative linear strands.[17][20] In any case, the sense of ssDNA viruses, unlike for ssRNA viruses, is not sufficient to separate ssDNA viruses into two groups since all ssDNA viral genomes are converted to dsDNA forms prior to transcription and replication.[3]

ssDNA viruses are classified into one of the four realms and include several families that are unassigned to a realm:

  • In Monodnaviria, all members except viruses in Papovaviricetes are ssDNA viruses.[14]
  • The unassigned families Anelloviridae and Spiraviridae are ssDNA virus families.[14]
  • Viruses in the family Finnlakeviridae contain ssDNA genomes. Finnlakeviridae is unassigned to a realm but is a proposed member of Varidnaviria.[13]

RNA viruses[edit]

Main articles: Orthornavirae and RNA virus

RNA viruses have genomes made of ribonucleic acid (RNA) and comprise three groups: double-stranded RNA (dsRNA) viruses, positive sense single-stranded RNA (+ssRNA) viruses, and negative sense single-stranded RNA (-ssRNA) viruses. The majority of RNA viruses are classified in the kingdom Orthornavirae in the realm Riboviria. The exceptions are generally viroids and other subviral agents, including Hepatitis D virus.

Group III: double-stranded RNA viruses[edit]

Rotaviruses are dsRNA viruses.

The third Baltimore group contains viruses that have a double-stranded RNA (dsRNA) genome. After entering a host cell, the dsRNA genome is transcribed to mRNA from the negative strand by the viral RNA-dependent RNA polymerase (RdRp). The mRNA may be used for translation or replication. Single-stranded mRNA is replicated to form the dsRNA genome. The 5'-end of the genome may be naked, capped, or covalently bound to a viral protein.[21][22]

dsRNA is not a molecule made by cells, so cellular life has evolved antiviral systems to detect and inactivate viral dsRNA. To counteract this, many dsRNA genomes are constructed inside of capsids, thereby avoiding detection inside of the host cell's cytoplasm. mRNA is forced out from the capsid in order to be translated or to be translocated from a mature capsid to a progeny capsid.[21][22][23] While dsRNA viruses typically have capsids, viruses in the families Amalgaviridae and Endornaviridae have not been observed to form virions and as such apparently lack capsids. Endornaviruses are also unusual in that unlike other RNA viruses, they possess a single, long open reading frame (ORF), or translatable portion, and a site-specific nick in the 5' region of the positive strand.[23]

dsRNA viruses are classified into two phyla within the kingdom Orthornavirae of the realm Riboviria:[24]

  • All viruses in Duplornaviricota are dsRNA viruses.
  • In Pisuviricota, all members of the class Duplopiviricetes are dsRNA viruses.

Group IV: positive sense single-stranded RNA viruses[edit]

Coronaviruses are +ssRNA viruses.

The fourth Baltimore group contains viruses that have a positive sense single-stranded RNA (+ssRNA) genome. For +ssRNA viruses, the genome functions as mRNA, so no transcription is required for translation. +ssRNA viruses will also, however, produce positive sense copies of the genome from negative sense strands of an intermediate dsRNA genome. This acts as both a transcription and a replication process since the replicated RNA is also mRNA. The 5'-end may be naked, capped, or covalently bound to a viral protein, and the 3'-end may be naked or polyadenylated.[25][26][27]

Many +ssRNA viruses are able to have only a portion of their genome transcribed. Typically, subgenomic RNA (sgRNA) strands are used for translation of structural and movement proteins needed during intermediate and late stages of infection. sgRNA transcription may occur by commencing RNA synthesis within the genome rather than from the 5'-end, by stopping RNA synthesis at specific sequences in the genome, or by, as a part of both prior methods, synthesizing leader sequences from the viral RNA that are then attached to sgRNA strands. Because replication is required for sgRNA synthesis, RdRp is always translated first.[26][27][28]

Because the process of replicating the viral genome produces intermediate dsRNA molecules, +ssRNA viruses can be targeted by the host cell's immune system. To avoid detection, +ssRNA viruses replicate in membrane-associated vesicles that are used as replication factories. From there, only viral +ssRNA, which may be mRNA, enters the main cytoplasmic area of the cell.[25][26]

+ssRNA viruses can be subdivided between those that have polycistronic mRNA, which encodes a polyprotein that is cleaved to form multiple mature proteins, and those that produce subgenomic mRNAs and therefore undergo two or more rounds of translation.[4][29] +ssRNA viruses are included in three phyla in the kingdom Orthornavirae in the realm Riboviria:[24]

  • All viruses in Lenarviricota are +ssRNA viruses.
  • All viruses in Pisuviricota are +ssRNA viruses, excluding the class Duplopiviricetes, whose members have dsRNA genomes.
  • All viruses in Kitrinoviricota are +ssRNA viruses.

Group V: negative sense single-stranded RNA viruses[edit]

Main article: Negarnaviricota

The fifth Baltimore group contains viruses that have a negative sense, single-stranded RNA (-ssRNA) genome. mRNA, which is positive sense, is transcribed directly from the negative sense genome. The first process for -ssRNA transcription involves RdRp binding to a leader sequence on the 3' end of the genome, transcribing a 5' triphosphate-leader RNA that is capped, then stopping and restarting on a transcription signal which is capped, continuing until a stop signal is reached.[30] The second manner is similar but instead of synthesizing a cap, RdRp may make use of cap snatching, whereby a short sequence of host cell mRNA is taken and used as the 5' cap of the viral mRNA.[31] Genomic -ssRNA is replicated from the positive sense antigenome in a similar manner as transcription, except in reverse using the antigenome as a template for the genome. RdRp moves from the 3'-end to the 5'-end of the antigenome and ignores all transcription signals when synthesizing genomic -ssRNA.[22][32]

Various -ssRNA viruses use special mechanisms for transcription. The manner of producing the polyA tail may be via polymerase stuttering, during which RdRp transcribes an adenine from uracil and then moves back in the RNA sequence with the mRNA to transcribe it again, continuing this process numerous times until hundreds of adenines have been added to the 3'-end of the mRNA.[33] Additionally, some -ssRNA viruses are ambisense, as both the positive and negative strands separately encode viral proteins, and these viruses produce two separate mRNA strands: one directly from the genome and one from a complementary strand.[34][35]

-ssRNA viruses can be subdivided informally between those that have nonsegmented and segmented genomes. Nonsegmented -ssRNA viruses replicate in the cytoplasm, and segmented -ssRNA viruses replicate in the nucleus. During transcription, the RdRp produces one monocistronic mRNA strand from each segment of the genome.[4][22][36] All -ssRNA viruses are classified in the phylum Negarnaviricota in the kingdom Orthornavirae in the realm Riboviria. Negarnaviricota only contains -ssRNA viruses, so "-ssRNA virus" is synonymous with Negarnaviricota.[24] Negarnaviricota is divided into two subphyla: Haploviricotina, whose members synthesize a cap structure on viral mRNA required for protein synthesis, and Polyploviricotina, whose members instead obtain caps on mRNA via cap snatching.[37]

Reverse transcribing viruses[edit]

Main article: Revtraviricetes

Reverse transcribing (RT) viruses have genomes made of either DNA or RNA and replicate via reverse transcription. Two groups of reverse transcribing viruses exist: single-stranded RNA-RT (ssRNA-RT) viruses, and double-stranded DNA-RT (dsDNA-RT) viruses. Reverse transcribing viruses are classified in the kingdom Pararnavirae in the realm Riboviria.

Group VI: single-stranded RNA viruses with a DNA intermediate[edit]

The sixth Baltimore group contains viruses that have a (positive-sense) single-stranded RNA genome that has a DNA intermediate ((+)ssRNA-RT) in its replication cycle.[note 1] ssRNA-RT viruses are transcribed in the same manner as DNA viruses, but their linear genomes are first converted to a dsDNA form through a process called reverse transcription. The viral reverse transcriptase enzyme synthesizes a DNA strand from the ssRNA strand, and the RNA strand is degraded and replaced with a DNA strand to create a dsDNA genome. The genome is then integrated into the DNA of the host cell, where it is now called a provirus. The host cell's RNA polymerase II then transcribes RNA in the nucleus from the proviral DNA. Some of this RNA may become mRNA whereas other strands will become copies of the viral genome for replication.[36][38][39][40]</ref>

ssRNA-RT viruses are all included in the class Revtraviricetes, phylum Arterviricota, kingdom Pararnavirae of the realm Riboviria. Excluding Caulimoviridae, which belongs to Group VII, all members of the Revtraviricetes order Ortervirales are ssRNA-RT viruses.[24][41]

Group VII: double-stranded DNA viruses with an RNA intermediate[edit]

The seventh Baltimore group contains viruses that have a double-stranded DNA genome that has an RNA intermediate (dsDNA-RT) in its replication cycle. dsDNA-RT viruses have a gap in one strand, which is repaired to create a complete dsDNA genome prior to transcription.[4][36] dsDNA-RT viruses are transcribed in the same manner as dsDNA viruses,[3] but make use of reverse transcription to replicate their circular genome while it is still in the capsid. The host cell's RNA polymerase II transcribes RNA strands from the genome in the cytoplasm, and the genome is replicated from these RNA strands. The dsDNA genome is produced from pregenomic RNA strands via the same general mechanism as ssRNA-RT viruses, but with replication occurring in a loop around the circular genome. After replication, the dsDNA genome may be packed or sent to the nucleus for further rounds of transcription.[38][42]

dsDNA-RT viruses are, like ssRNA-RT, all included in the class Revtraviricetes. Two families of dsDNA-RT viruses are recognized: Caulimoviridae, which belongs to the order Ortervirales, and Hepadnaviridae, which is the sole family in the order Blubervirales.[24][41]

Multi-group characteristics[edit]

Structure of some viruses classified by Baltimore group: HSV (group I), HCV (group IV), DENV (group IV), IAV (group V), and HIV-1 (group VI).

A number of characteristics of viruses are not directly associated with Baltimore classification but nonetheless closely correspond to multiple, specific Baltimore groups. This includes alternative splicing during transcription, whether the viral genome is segmented, the host range of viruses, whether the genome is linear or circular, and different methods of translating viral mRNA.

Alternative splicing[edit]

Alternative splicing is a mechanism by which different proteins can be produced from a single gene by means of using alternative splicing sites to produce different mRNAs. It is found in various DNA, -ssRNA, and reverse transcribing viruses. Viruses may make use of alternative splicing solely to produce multiple proteins from a single pre-mRNA strand or for other specific purposes. For certain viruses, including the families Orthomyxoviridae and Papillomaviridae, alternative splicing acts as a way to regulate early and late gene expression during different stages of infection. Herpesviruses use it as a potential anti-host defense mechanism to prevent synthesis of specific antiviral proteins. Furthermore, in addition to alternative splicing, because cellular unspliced RNA cannot be transported out of the nucleus, hepadnaviruses and retroviruses contain their own proteins for exporting their unspliced genomic RNA out of the nucleus.[43][44]

Genome segmentation[edit]

Viral genomes can exist in a single, or monopartite, segment, or they may be split into more than one molecule, called multipartite. For monopartite viruses, all genes are on the single segment of the genome. Multipartite viruses typically package their genomes into a single virion so that the whole genome is in one virus particle, and the separate segments contain different genes. Monopartite viruses are found in all Baltimore groups, whereas multipartite viruses are usually RNA viruses. This is because most multipartite viruses infect plants or fungi, which are eukaryotes, and most eukaryotic viruses are RNA viruses.[45][46][47] The family Pleolipoviridae varies as some viruses are monopartite ssDNA while others are bipartite with one segment being ssDNA and the other dsDNA.[7][48] Viruses in the ssDNA plant virus family Geminiviridae likewise vary between being monopartite and bipartite.[46][49]

Host range[edit]

Different Baltimore groups tend to be found within different branches of cellular life. In prokaryotes, the large majority of viruses are dsDNA viruses, and a significant minority are ssDNA viruses. Prokaryotic RNA viruses, in contrast, are relatively rare. Most eukaryotic viruses, including most animal and plant viruses, are RNA viruses, although eukaryotic DNA viruses are also common. By group, the vast majority of dsDNA viruses infect prokaryotes, ssDNA viruses are found in all three domains of life, dsRNA and +ssRNA viruses are primarily found in eukaryotes but also in bacteria, and -ssRNA and reverse transcribing viruses are only found in eukaryotes.[46][45][50]

Linear vs circular genomes[edit]

Viral genomes may be either linear with ends or circular in a loop. Whether a virus has a linear or circular genome varies from group to group. A significant percentage of dsDNA viruses are both, ssDNA viruses are primarily circular, RNA viruses and ssRNA-RT viruses are typically linear, and dsDNA-RT viruses are typically circular.[51][52] In the dsDNA family Sphaerolipoviridae, and in the family Pleolipoviridae, viruses contain both linear and circular genomes, varying from genus to genus.[7][48][53]

RNA editing[edit]

RNA editing is used by various ssRNA viruses to produce different proteins from a single gene. This can be done via polymerase slippage during transcription or by post-transcriptional editing. In polymerase slippage, the RNA polymerase slips one nucleotide back during transcription, inserting a nucleotide not included in the template strand. Editing of a genomic template would impair gene expression, so RNA editing is only done during and after transcription. For ebola viruses, RNA editing improves the ability to adapt to their hosts.[44][54]

Alternative splicing differs from RNA editing in that alternative splicing does not change the mRNA sequence like RNA editing but instead changes the coding capacity of an mRNA sequence as a result of alternative splicing sites. The two mechanisms otherwise have the same result: multiple proteins are expressed from a single gene.[44]

Translation[edit]

Life cycle of some viruses classified by Baltimore group: HSV (group I), HCV (group IV), IAV (group V), and HIV-1 (group VI).

Translation is the process by which proteins are synthesized from mRNA by ribosomes. Baltimore groups do not directly pertain to the translation of viral proteins, but various atypical types of translation used by viruses are usually found within specific Baltimore groups:[3][55]

  • Non-canonical translation initiation:
    • Viral initiation of translation: used primarily by +ssRNA and ssRNA-RT viruses, various viruses have evolved mechanisms to initiate translation, such as having internal ribosomal entry sites to allow for cap-independent translation, having downstream hairpin loops that allow for cap-dependent translation in the absence of an eIF2 initiation factor, and initiation at a CUG or other start codon with a leucine amino acid.[56][57]
    • Leaky scanning: used by various viruses in all Baltimore groups, the 40S ribosomal subunit may scan through a start codon, thereby skipping an ORF, only initiating translation with the 60S subunit at a subsequent start codon.[58][59]
    • Ribosomal shunting: used by various dsDNA, +ssRNA, -ssRNA, ssRNA-RT, a dsDNA-RT viruses, ribosomes will start scanning from a 5'-cap structure then bypass a leader structure in the mRNA, initiation translation downstream from the leader sequence.[60][61]
    • Termination-reinitiation: used by some dsRNA and +ssRNA viruses, ribosomes may translate an ORF, but following termination of translation of that ORF, a proportion of 40S subunits of the ribosome remain attached to the mRNA as a way to reinitiate translation of a subsequent ORF.[62]
  • Non-canonical elongation and termination of translation:
    • Ribosomal frameshifting: used by various dsDNA, dsRNA, +ssRNA, and ssRNA-RT viruses, produces merged proteins from overlapping ORFs. This is executed simply by ribosomes slipping one nucleobase forward or backward during translation.[59][63]
    • Suppression of termination: also called stop-codon readthrough, used by various dsRNA, +ssRNA, and ssRNA-RT viruses, certain viruses contain codons in their mRNA that would normally signal for termination of translation upon being recognized by a release factor but are instead partially recognized by tRNA during translation, which allows for continued translation up to the next stop codon in order to produce an extended end of the viral protein.[64] In viruses, this is often used to express replicase enzymes.[65]
    • Ribosomal skipping: also called stop-carry on, used by various dsRNA and +ssRNA viruses, a viral peptide, or amino acid sequence, may prevent a ribosome from covalently linking a new inserted amino acid, which blocks further translation. Consequently, the polyprotein is co-translationally cleaved, and a new amino acid sequence is started, leading to the production of two individual proteins from one ORF.[61][66]

History[edit]

David Baltimore

Baltimore classification was proposed in 1971 by virologist David Baltimore in a paper titled Expression of Animal Virus Genomes. It initially contained the first six groups but was later expanded to include group VII.[36][67][68] Because of the utility of Baltimore classification, it has come to be used alongside standard virus taxonomy, which is based on evolutionary relationships and governed by the International Committee on Taxonomy of Viruses (ICTV).[68]

From the 1990s to the 2010s, virus taxonomy used a 5-rank system ranging from order to species with Baltimore classification used in conjunction. Outside of the ICTV's official framework, various supergroups of viruses joining together different families and orders were created over time based on increasing evidence of deeper evolutionary relations. Consequently, in 2016, the ICTV began to consider establishing ranks higher than order as well as how the Baltimore groups would be treated among higher taxa.[68]

In two votes in 2018 and 2019, a 15-rank system ranging from realm to species was established by the ICTV.[68] As part of this, the Baltimore groups for RNA viruses and RT viruses were incorporated into formal taxa. In 2018, the realm Riboviria was established and initially included the three RNA virus groups.[69] A year later, Riboviria was expanded to also include both RT groups. Within the realm, RT viruses are included in the kingdom Pararnavirae and RNA viruses in the kingdom Orthornavirae. Furthermore, the three Baltimore groups for RNA viruses are used as defining characteristics of the phyla in Orthornavirae.[24]

Unlike RNA viruses and RT viruses, DNA viruses have not been united under a single realm but are instead dispersed across three realms and various taxa that are not assigned to a realm. The realm Duplodnaviria exclusively contains dsDNA viruses,[12] Monodnaviria primarily contains ssDNA viruses but also contains dsDNA viruses,[14] and Varidnaviria exclusively contains dsDNA viruses, although some proposed members of Varidnaviria, namely the family Finnlakeviridae, are ssDNA viruses.[13]

Notes[edit]

  1. ^ ssRNA-RT viruses are often called retroviruses, although this term is also used to refer to any reverse transcribing virus as well as specifically to viruses in the ssRNA-RT family Retroviridae.

References[edit]

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