The Two Simplest RNA Viruses

Narnaviridae

Learn more from the articles linked below:

• Family - Narnaviridae from ScienceDirect

• Narnaviridae from ScienceDirect

• Narnavirus from ScienceDirect

• ICTV 9th Report (2011) about Narnaviridae

• Mitovirus from Mitovirus

Taxonomy

The Narnaviridae family is subdivided into two genera: Narnavirus and Mitovirus. Members of the genus Narnavirus have been found in Saccharomyces cerevisiae (yeast) and in Phytophthora infestans (algae). Members of the genus Mitovirus have been found in filamentous fungi and replicate in host mitochondria. Narnavirus species include: Saccharomyces 20S RNA narnavirus and Saccharomyces 23S RNA narnavirus. Mitovirus species include: Cryphonectria mitovirus 1, Ophiostoma mitovirus 4, Ophiostoma mitovirus 5, Ophiostoma mitovirus 6, and Ophiostoma mitovirus 3a.

Genomic Organization and Replication

Members of the Narnaviridae virus family contain the simplest genomes of any RNA virus. Their single-stranded, linear, positive-sense RNA genomes range from approximately 2.5 kb to 2.9 kb, with a single ORF encoding an RNA-dependent RNA polymerase (RdRp) domain of 80-104kDA. Viruses of this family do not encode structural proteins, and like most RNA viruses of lower eukaryotes, their genomes are associated with the RdRp confined within lipid vesicles in the cytoplasm. Their virion RNA genome transcribes and translates to produce viral, infectious RdRp. The putative RdRp proteins of narnaviruses and mitoviruses are distantly related to those of bacteriophages in the family Leviviridae.

Narnavirus:

Since Narnaviruses do not encode capsid genes and proteins, their genomes are not encapsidated within intracellular virions. Therefore, their genomes, associated with their RdRps, form ribonucleoprotein complexes in a stoichiometry of 1:1 and reside in the cytoplasm. Their simple replication cycle provides the molecular basis and persistence of other RNA viruses in Eukaryotes.

Mitovirus:

Mitoviral RdRps contain internal UGA stop codons, which encode the amino acid Tryptophan as long as the RdRps are active in the mitochondria. The property of fungal mitochondria helps the mitovirus replicate by producing a polypeptide of 80 - 97 kDa, containing RdRp motifs.

Origin and Evolution

Narnavirus:

Narnaviruses escaped from the mitochondria to the cytosol of Eukaryotic cells. Their descendants became virulent by acquiring the gene encoding SJR-CP, which are viral capsid proteins believed to have given rise to virus evolution, as well as a movement protein and helicase.

Mitovirus:

Mitoviruses are considerably diverse and are currently evolving via horizontal transfer from one species to another distant species. Though mainly found in fungi, mitoviruses have recently been found in plant mitochondria.

• ScienceDirect. Chapter Six - The Family Narnaviridae: Simplest of RNA Viruses, https://www.sciencedirect.com/science/article/pii/B9780123943156000064
• ViralZone. Narnaviridae, https://viralzone.expasy.org/46?outline=all_by_species
• ICTV, https://talk.ictvonline.org/ictv-reports/ictv_9th_report/positive-sense-rna-viruses-2011/w/posrna_viruses/268/narnaviridae-figures
• ScienceDirect. Narnavirus, https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/narnavirus
• ScienceDirect. Narnaviridae, https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/narnaviridae
• ScienceDirect. Mitovirus, https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/mitovirus