RNA virus
An RNA virus is a virus—other than a retrovirus—that has ribonucleic acid (RNA) as its genetic material.[1] The nucleic acid is usually single-stranded RNA (ssRNA) but it may be double-stranded (dsRNA).[2] 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.
The International Committee on Taxonomy of Viruses (ICTV) classifies RNA viruses as those that belong to Group III, Group IV or Group V of the Baltimore classification system. This category excludes Group VI, viruses with RNA genetic material but which use DNA intermediates in their life cycle: these are called retroviruses,[3] including HIV-1 and HIV-2 which cause AIDS.
As of May 2020, all known RNA viruses encoding an RNA-directed RNA polymerase are believed to form a monophyletic group, known as the realm Riboviria.[4] The majority of such RNA viruses fall into the kingdom Orthornavirae and the rest have a positioning not yet defined.[5] The realm does not contain all RNA viruses: Deltavirus, Asunviroidae, and Pospiviroidae are taxa of RNA viruses that were mistakenly included in 2019,[a] but corrected in 2020.[6]
Characteristics[edit]
Single-stranded RNA viruses and RNA Sense[edit]
RNA viruses can be further classified according to the sense or polarity of their RNA into negative-sense and positive-sense, or ambisense RNA viruses. Positive-sense viral RNA is similar to mRNA and thus can be immediately translated by the host cell. Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA-dependent RNA polymerase before translation. Purified RNA of a positive-sense virus can directly cause infection though it may be less infectious than the whole virus particle. In contrast, purified RNA of a negative-sense virus is not infectious by itself as it needs to be transcribed into positive-sense RNA; each virion can be transcribed to several positive-sense RNAs. Ambisense RNA viruses resemble negative-sense RNA viruses, except they translate genes from their negative and positive strands.[7]
Animal RNA viruses are classified by the ICTV. There are three distinct groups of RNA viruses depending on their genome and mode of replication:
Retroviruses (Group VI) have a single-stranded RNA genome but, in general, are not considered RNA viruses because they use DNA intermediates to replicate. Reverse transcriptase, a viral enzyme that comes from the virus itself after it is uncoated, converts the viral RNA into a complementary strand of DNA, which is copied to produce a double-stranded molecule of viral DNA. After this DNA is integrated into the host genome using the viral enzyme integrase, expression of the encoded genes may lead to the formation of new virions.
Recombination[edit]
Numerous RNA viruses are capable of genetic recombination when at least two viral genomes are present in the same host cell.[16] Very rarely viral RNA can recombine with host RNA.[17] RNA recombination appears to be a major driving force in determining genome architecture and the course of viral evolution among Picornaviridae ((+)ssRNA), e.g. poliovirus.[18] In the Retroviridae ((+)ssRNA), e.g. HIV, damage in the RNA genome appears to be avoided during reverse transcription by strand switching, a form of recombination.[19][20][21] Recombination also occurs in the Reoviridae (dsRNA), e.g. reovirus; Orthomyxoviridae ((-)ssRNA), e.g. influenza virus;[21] and Coronaviridae ((+)ssRNA), e.g. SARS.[22] Recombination in RNA viruses appears to be an adaptation for coping with genome damage.[16] Recombination can occur infrequently between animal viruses of the same species but of divergent lineages. The resulting recombinant viruses may sometimes cause an outbreak of infection in humans.[22]
There are twelve families and a number of unassigned genera and species recognised in this group.[10]
There are three orders and 34 families recognised in this group. In addition, there are a number of unclassified species and genera.
Satellite viruses
An unclassified astrovirus/hepevirus-like virus has also been described.[36]
With the exception of the Hepatitis D virus, this group of viruses has been placed into a single phylum—Negarnaviricota. This phylum has been divided into two subphyla—Haploviricotina and Polyploviricotina. Within the subphylum Haploviricotina four classes are currently recognised: Chunqiuviricetes, Milneviricetes, Monjiviricetes and Yunchangviricetes. In the subphylum Polyploviricotina two classes are recognised: Ellioviricetes and Insthoviricetes.
Six classes, seven orders and twenty four families are currently recognized in this group. A number of unassigned species and genera are yet to be classified.[10]
