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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Baltimore classification | 3/7 | https://en.wikipedia.org/wiki/Baltimore_classification | reference | science, encyclopedia | 2026-05-05T09:06:57.533227+00:00 | kb-cron |
Viruses in the phylum Duplornaviricota are dsRNA viruses. In the phylum Pisuviricota, members of the class Duplopiviricetes are dsRNA viruses.
=== Group IV: positive-sense single-stranded RNA 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, however, produce positive-sense copies of the genome from negative-sense strands of an intermediate dsRNA genome. This acts as both a transcription and replication process since the replicated +ssRNA is also mRNA. Many +ssRNA viruses are able to have only a portion of their genome transcribed. Typically, subgenomic RNA (sgRNA) strands are used for the 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 ("five prime end"), by stopping RNA synthesis at specific sequences in the genome, or, as a part of both aforementioned methods, by synthesizing leader sequences from viral RNA that are then attached to sgRNA strands. During infection, the viral RdRp is always translated directly from the genome first because replication, performed by the RdRp, is required for sgRNA synthesis. 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 +ssRNA strands enter the main cytoplasmic area of the cell. These strands may be used as mRNA or as progeny genomes. +ssRNA viruses can be divided informally into those that have polycistronic mRNA, which encodes a polyprotein that is cleaved to form multiple mature proteins, and those that undergo multiple rounds of translation of the genome or subgenomic mRNAs to express proteins. +ssRNA viruses are classified into three phyla in the kingdom Orthornavirae, realm Riboviria:
Viruses in the phyla Kitrinoviricota and Lenarviricota are +ssRNA viruses. Viruses in the phylum Pisuviricota are +ssRNA viruses, excluding the class Duplopiviricetes, which contains dsRNA viruses.
=== Group V: negative-sense single-stranded RNA viruses ===
The fifth Baltimore group contains viruses that have a negative-sense, single-stranded RNA (–ssRNA) genome. At least two lineages of –ssRNA viruses exist, which transcribe and replicate their genomes differently. The first are viruses of the phylum Negarnaviricota in the kingdom Orthornavirae, realm Riboviria. Negarnaviricots transcribe mRNA, which is positive sense, directly from the negative-sense genome. The first process for –ssRNA transcription involves the viral RdRp binding to a leader sequence on the 3′-end of the genome, transcribing a 5′ triphosphate-leader RNA sequence, then stopping and restarting on a transcription signal that is capped, continuing until a stop signal is reached. There, the RdRp synthesizes a polyadenylated tail and releases the mRNA or, for polycistronic genomes, continues transcription. The second manner is similar, but instead of synthesizing a cap, the RdRp may use its endonuclease activity to snatch a short sequence of nucleotides from host cell mRNA and use it as the 5′ cap of viral mRNA. Genomic –ssRNA is replicated from the positive-sense antigenome in a manner similar to transcription, except in reverse using the antigenome as a template for the genome. The RdRp complex moves from the 3′-end to the 5′-end of the antigenome and ignores all transcription signals when synthesizing genomic –ssRNA. Various –ssRNA viruses use special mechanisms for transcription. The way of polyadenylating the end of an mRNA sequence may be through polymerase stuttering, during which the RdRp transcribes an adenine from uracil and then moves back in the RNA sequence to transcribe it again, continuing this process until hundreds of adenines have been added to the 3′-end of the mRNA. Some –ssRNA viruses are ambisense, as both the positive- and negative-sense strands separately encode viral proteins. These viruses produce one mRNA strand from the genome and one from a complementary strand. –ssRNA viruses in Negarnaviricota can be divided informally into those that have non-segmented and segmented genomes. Non-segmented –ssRNA viruses replicate in the cytoplasm, and segmented –ssRNA viruses replicate in the nucleus. For segmented viruses, the RdRp transcribes one monocistronic mRNA strand from each segment of the genome. This distinction is closely followed within Negarnaviricota, as viruses in the subphylum Haploviricotina usually have non-segmented genomes, and viruses in the subphylum Polyploviricotina have segmented genomes. Moreover, –ssRNA viruses that synthesize a cap structure on viral mRNA are assigned to Haploviricotina, whereas –ssRNA viruses that snatch caps from host mRNA belong to Polyploviricotina.
The second lineage of –ssRNA viruses is the realm Ribozyviria, which includes Hepatitis D virus (HDV) and its relatives. Ribozyvirians have covalently-closed circular –ssRNA genomes that are covered in nucleocapsid proteins to form a ribonucleoprotein (RNP) complex. After entering a cell, the RNP complex migrates from the cytosol to the nucleus, where the genome is replicated by RCR by a host RNA polymerase II enzyme. This process creates a long molecule with many copies of the genome, called a concatemer, that has a series of positive-sense genomic strands. Ribozymes encoded in this antigenome catalyze cleavage of the concatemer to form individual strands that are either translated or ligated for replication through RCR to produce concatemers of –ssRNA antigenomic strands. Ribozymes encoded in the negative-sense strands then catalyze cleavage of the negative-sense concatemer to produce individual genomic –ssRNA strands. Lastly, there is a group of –ssRNA viruses assigned to the tentative phylum Arctiviricota in the kingdom Orthornavirae. Arctiviricots inhabit the Arctic Ocean and are believed to represent a separate –ssRNA lineage in Orthornavirae from Negarnaviricota. Their mechanisms of replication and transcription have not been described. In summary, –ssRNA viruses belong to the following taxa:
In the realm Riboviria, viruses in the phyla Arctiviricota (tentative) and Negarnaviricota are –ssRNA viruses. Viruses in the realm Ribozyviria are –ssRNA viruses.
=== Group VI: single-stranded RNA viruses with a DNA intermediate ===