Positivesense

Positive-sense RNA, often abbreviated as positive-sense or (+)RNA, refers to a class of single-stranded RNA molecules whose sequence can function directly as messenger RNA in a host cell. In virology, positive-sense RNA viruses carry genomes that are ready for translation by host ribosomes upon entry, so their genomes can be immediately used to synthesize viral proteins without initial transcription. In contrast, negative-sense RNA viruses carry genomes that are complementary to mRNA and require viral RNA-dependent RNA polymerase to synthesize a positive-sense intermediate before translation.

Replication cycle: after entering the cell, the viral RNA acts as mRNA and a replication template. The virus encodes or carries an RNA-dependent RNA polymerase to synthesize a complementary negative-sense strand, which then serves as a template to produce new positive-sense genome copies. Some positive-sense RNA viruses also replicate in association with membranous compartments in the cytoplasm, forming replication complexes.

Examples: well-known positive-sense RNA virus families include Picornaviridae (poliovirus, rhinovirus), Flaviviridae (hepatitis C virus, dengue virus), Coronaviridae (common cold coronaviruses, SARS-CoV-2), and Togaviridae (chikungunya virus). The genome size and organization vary; some have polyproteins that are processed into mature viral proteins, others encode separate structural and nonstructural proteins.

Clinical and research relevance: detection of positive-sense viral RNA by RT-qPCR is standard for diagnosing infections. In cellular contexts beyond virology, the term positive-sense describes RNAs whose polarity matches messenger RNA, enabling direct translation by ribosomes.