Virusoids are circular single-stranded RNAs dependent on plant viruses for replication and encapsidation. The genome of virusoids consist of several hundred nucleotides and does not code for any proteins.
Virusoids are essentially viroids that have been encapsidated by a helper virus coat protein. Therefore, they are similar to viroids in terms of their means of replication (Rolling circle replication), but differ in that viroids do not possess a protein coat.
Virusoids, while being studied in virology, are not considered as viruses but as subviral particles. Since they depend on helper viruses, they are classified as satellites. In the virological taxonomy they appear as Satellites/Satellite nucleic acids/Subgroup 3: Circular satellite RNAs.
The term virusoid is also sometimes used more generally to refer to all satellites.
- Symons RH (1991). "The intriguing viroids and virusoids: what is their information content and how did they evolve?" (PDF). Mol. Plant Microbe Interact. 4 (2): 111–21. PMID 1932808. doi:10.1094/MPMI-4-111.
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