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|Positive ssRNA Virus|
|Hepatitis C virus|
|Group:||Group IV ((+)ssRNA)|
|Orders, Families, and Genera|
A positive-sense single-stranded RNA virus (or (+)ssRNA virus) is a virus that uses positive sense, single-stranded RNA as its genetic material. Single stranded RNA viruses are classified as positive or negative depending on the sense or polarity of the RNA. The positive-sense viral RNA genome can also serve as messenger RNA and can be translated into protein in the host cell. Positive-sense ssRNA viruses belong to Group IV in the Baltimore classification. Positive-sense RNA viruses account for a large fraction of known viruses, including many pathogens such as the hepatitis C virus, West Nile virus, dengue virus, and SARS and MERS coronaviruses, as well as less clinically serious pathogens such as the rhinoviruses that cause the common cold.
Positive-sense ssRNA viruses have genetic material that can function both as a genome and as messenger RNA; it can be directly translated into protein in the host cell by host ribosomes. The first proteins to be expressed after infection serve genome replication functions; they recruit the positive-strand viral genome to viral replication complexes (VRCs) formed in association with intracellular membranes. VRCs contain proteins of both viral and host cell origin, and may be associated with the membranes of a variety of organelles, often the rough endoplasmic reticulum, but also including membranes derived from mitochondria, vacuoles, the Golgi apparatus, chloroplasts, peroxisomes, plasma membranes, autophagosomal membranes, and novel cytoplasmic compartments. The replication of the positive-sense ssRNA genome proceeds through double-stranded RNA intermediates, and the purpose of replication in these membranous invaginations may be the avoidance of cellular response to the presence of dsRNA. In many cases subgenomic RNAs are also created during replication. After infection, the entirety of the host cell's translation machinery may be diverted to the production of viral proteins as a result of the very high affinity for ribosomes of the viral genome's internal ribosome entry site (IRES) elements; in some viruses, such as poliovirus and rhinoviruses, normal protein synthesis is further disrupted by viral proteases degrading components required to initiate translation of cellular mRNA.
All positive-sense ssRNA virus genomes encode RNA-dependent RNA polymerase (RdRP), a viral protein that synthesizes RNA from an RNA template. Host cell proteins recruited by positive-sense ssRNA viruses during replication include RNA-binding proteins, chaperone proteins, and membrane remodeling and lipid synthesis proteins, which collectively participate in exploiting the cell's secretory pathway for viral replication.
The genome of a positive-sense ssRNA virus usually contains relatively few genes, usually between three and ten, including an RdRP. Coronaviruses have the largest known RNA genomes, up to 32 kilobases in length, and likely possess replication proofreading mechanisms in the form of a proofreading exoribonuclease, non-structural protein 14, that is otherwise not found in RNA viruses.
The (+)ssRNA viruses are classified into 3 orders — the Nidovirales, Picornavirales, and Tymovirales — and 33 families, of which 20 are not assigned to an order. A broad range of hosts can be infected by (+)ssRNA viruses, including bacteria (the Leviviridae), eukaryotic microorganisms, plants, invertebrates, and vertebrates. No examples have been described that infect archaea, whose virome is generally much less well-characterized.
Among known (+)ssRNA viruses, only the Leviviridae are bacteriophages (that is, viruses that infect bacteria). Known leviviruses infect enterobacteria. Phage with RNA genomes are relatively rare and poorly understood, with only one other recognized group — a family of double-stranded RNA viruses called the Cystoviridae. However, metagenomics has led to the identification of numerous additional novel examples.
Positive-sense ssRNA viruses are the most common type of plant virus. Members of the (+)ssRNA picornavirus group are also extremely abundant — to the point of "unexpected dominance" — in marine viruses characterized by metagenomics. These viruses likely infect single-celled eukaryotes.
There are eight families of (+)ssRNA viruses that infect vertebrates, of which four are unenveloped (Picornaviridae, Astroviridae, Caliciviridae, and Hepeviridae) and four are enveloped (Flaviviridae, Togaviridae, Arteriviridae, and Coronaviridae). All but the arterivirus family contain at least one human pathogen; arteriviruses are known only as animal pathogens. Many pathogenic (+)ssRNA viruses are arthropod-borne viruses (also called arboviruses) — that is, transmitted by and capable of replicating in biting insects which then transfer the pathogen to animal hosts. Recent metagenomics studies have also identified large numbers of RNA viruses whose host range is specific to insects.