ssRNAgenom

ssRNAgenom refers to a genome composed of single-stranded RNA. In virology and molecular biology, ssRNA genomes are found in many RNA viruses and in some plant or animal viroids. The single-stranded nature influences how the genome must be used and replicated inside a host cell.

A key distinction is genome sense. Positive-sense ssRNA (+ssRNA) can serve directly as messenger RNA in the host cell and be translated into viral proteins. Negative-sense ssRNA (-ssRNA) is complementary to mRNA and must first be copied into a positive-sense template by an RNA-dependent RNA polymerase, which is usually packaged in the virion. A subset of ssRNA viruses uses a reverse-transcribed life cycle, where +ssRNA serves as the genome but is reverse-transcribed into DNA and integrated; these are classified as RNA-positives with reverse transcription (reverse-transcribing viruses). Some ssRNA viruses have segmented genomes, allowing reassortment of segments during coinfection, which can drive genetic diversity.

Replication and transcription strategies vary. +ssRNA viruses often translate replication proteins immediately after infection, with replication occurring in the cytoplasm. -ssRNA viruses typically carry the polymerase in the virion to synthesize mRNA and genome copies in the cytoplasm. The replication machinery of many RNA viruses is error-prone, leading to high mutation rates and rapid evolution, which affects pathogenicity, host range, and immune evasion.

Common examples include +ssRNA viruses such as coronaviruses and flaviviruses, as well as -ssRNA viruses such as orthomyxoviruses and paramyxoviruses. Retroviruses, though an exception in their life cycle, use +ssRNA as their genome and rely on reverse transcription. Understanding ssRNA genomes informs vaccine design, antiviral targets, and studies of viral evolution.