Avsunviroidae

The Avsunviroidae are a family of viroids. There are four species in three genera (Avsunviroid, Elaviroid and Pelamoviroid).[1] They consist of RNA genomes between 246-375 nucleotides in length. They are single-stranded covalent circles and have intramolecular base pairing. All members lack a central conserved region.[2]

Avsunviroidae
Virus classification
(unranked): Virus
Realm: incertae sedis
Kingdom: incertae sedis
Phylum: incertae sedis
Class: incertae sedis
Order: incertae sedis
Family: Avsunviroidae
Genera

Replication

Replication occurs in the chloroplasts of plant cells. Key features of replication include no helper virus required and no proteins are encoded for. Unlike the other family of viroids, Pospiviroidae, Avsunviroidae are thought to replicate via a symmetrical rolling mechanism. It is thought the positive RNA strand acts as a template to form negative strands with the help of an enzyme thought to be RNA polymerase plus 3 II. The negative RNA strands are then cleaved by ribozyme activity and circularises. A second rolling circle mechanism forms a positive strand which is also cleaved by ribozyme activity and then ligated to become circular. The site of replication is unknown but it is thought to be in the chloroplast and in the presence of Mg2+ ions.[2]

Structure

Predictions of structure have suggested that they exist either as rod-shaped molecules with regions of base pairing causing formation of some hair pin loops or have branched configurations.[2][3]

The family has four stretches of conserved nucleotides, guuuc, uc, ucag, ac from 5' to 3', plus their Watson-Crick pairings on the other end of the loop. This is part of its hammerhead ribozyme.[1]

Classification

The family has three genera, with a total of five species.[2]

Detection

The lack of a long, central conserved region makes Avsunviroidae harder to identify than Pospiviroidae. A method to detect them is to use their circularity: a computer can piece together many overlapping reads that appear to form repeats when placed linearly.[4]

References
  1. Di Serio, F; Li, SF; Matoušek, J; Owens, RA; Pallás, V; Randles, JW; Sano, T; Verhoeven, JTJ; Vidalakis, G; Flores, R; Ictv Report, Consortium (May 2018). "ICTV Virus Taxonomy Profile: Avsunviroidae". The Journal of General Virology. 99 (5): 611–612. doi:10.1099/jgv.0.001045. PMID 29580320.
  2. "ICTV Online Report Avsunviroidae".
  3. Giguère, T; Adkar-Purushothama, CR; Bolduc, F; Perreault, JP (October 2014). "Elucidation of the structures of all members of the Avsunviroidae family". Molecular Plant Pathology. 15 (8): 767–79. doi:10.1111/mpp.12130. PMC 6638799. PMID 25346967.
  4. Wu, Q; Wang, Y; Cao, M; Pantaleo, V; Burgyan, J; Li, WX; Ding, SW (6 March 2012). "Homology-independent discovery of replicating pathogenic circular RNAs by deep sequencing and a new computational algorithm". Proceedings of the National Academy of Sciences of the United States of America. 109 (10): 3938–43. Bibcode:2012PNAS..109.3938W. doi:10.1073/pnas.1117815109. PMC 3309787. PMID 22345560.
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