Positive-strand RNA virus

Positive-strand RNA viruses (+ssRNA 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.

Positive-strand RNA viruses are divided between the phyla Kitrinoviricota, Lenarviricota, and Pisuviricota (specifically classes Pisoniviricetes and Stelpavirictes) all of which are in the kingdom Orthornavirae and realm Riboviria. They are monophyletic and descended from a common RNA virus ancestor. In the Baltimore classification system, +ssRNA viruses belong to Group IV.

Positive-sense RNA viruses include pathogens such as the Hepatitis C virus, West Nile virus, dengue virus, and the MERS, SARS, and SARS-CoV-2 coronaviruses, as well as less clinically serious pathogens such as the coronaviruses and rhinoviruses that cause the common cold.

Genome

Positive-strand RNA virus genomes usually contain relatively few genes, usually between three and ten, including an RNA-dependent RNA polymerase. Coronaviruses have the largest known RNA genomes, between 27 and 32 kilobases in length, and likely possess replication proofreading mechanisms in the form of an exoribonuclease within nonstructural protein nsp14.

Replication

Positive-strand RNA 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 formed in association with intracellular membranes.