# Virusoid
A virusoid is a small, circular, single-stranded RNA molecule that behaves as a satellite of a plant virus: it is encapsidated by its helper virus, replicated by the viral polymerase, and cannot multiply on its own. The name was coined for the viroid-like satellite RNAs of sobemoviruses, in which the circular form is the predominant RNA packaged in virus particles, while the linear form predominates in the viroid-like satellite RNAs of nepoviruses and luteoviruses.[1](https://www.tandfonline.com/doi/pdf/10.4161/rna.8.2.14238?needAccess=true) In broader usage, satellite viruses or virusoids are small RNA or DNA molecules coding for one or two proteins whose replication and spread depend on the presence of another virus.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC7123905/)
| Key fact | Detail |
|---|---|
| Definition | Small, circular, single-stranded satellite RNAs encapsidated by plant viruses and replicated by the viral polymerase[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10975798/) |
| Dependence | Depend on viruses to provide the proteins for their replication and protein coat[4](https://accesspharmacy.mhmedical.com/content.aspx?bookid=3541§ionid=294615601) |
| Structure | Genome resembles a viroid: circular ssRNA with self-cleaving ribozyme activity and a mostly rod-like secondary structure with a few small bifurcations[5](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/satellite-virus) • [3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10975798/) |
| Origin of the term | Coined for the viroid-like satellite RNAs of sobemoviruses, where the circular form is the predominant encapsidated RNA[1](https://www.tandfonline.com/doi/pdf/10.4161/rna.8.2.14238?needAccess=true) |
| Medical example | Hepatitis D virus proliferates only in cells simultaneously infected with hepatitis B virus and is described as a virusoid[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC7123905/) |
| Evolutionary interest | Viroids and virusoids, a class of satellite RNAs, have been proposed as molecular fossils of the RNA world that preceded the DNA-and-protein world[6](https://www.scielo.br/j/fb/a/tbP38Cwhvvd4yBQqKRctGQJ/?format=html&ilang=pt_BR) |
## Definition and dependence on a helper virus
Virusoids belong to the wider group of [subviral agents](https://www.edgepedia.org/subviral-agent), infectious entities simpler than viruses. Until the middle of the twentieth century viruses were considered the simplest biological entities; the discovery of satellite RNAs and viroids around 1970 revealed a class of self-replicating molecules even simpler, called subviral agents.[6](https://www.scielo.br/j/fb/a/tbP38Cwhvvd4yBQqKRctGQJ/?format=html&ilang=pt_BR) Satellite RNAs are similar to viroids in size, structure, and pathological effects, but they do not replicate autonomously; they need a specific helper virus to multiply.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10975798/)
The dependence is complete. Harrison's *Principles of Internal Medicine* defines virusoids as small, circular-RNA infectious pathogens that depend on viruses to provide the proteins for their replication and protein coat, in contrast to viroids, which are small circular single-stranded RNA pathogens of plants without a protein coat.[4](https://accesspharmacy.mhmedical.com/content.aspx?bookid=3541§ionid=294615601) [Satellite](https://www.edgepedia.org/satellite) viruses more generally are subviral particles whose DNA or RNA genome codes for a capsid protein but which are absolutely dependent on another virus for replication; the vast majority are found among plants.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC7173592/)
## Structure and replication
The virusoid genome resembles a viroid: it is circular single-stranded RNA with self-cleaving ribozyme activity.[5](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/satellite-virus) Its native secondary structure is mostly rod-like with a few small bifurcations, and the molecule is encapsidated by the respective plant virus and replicated by the viral polymerase.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10975798/)
Replication follows a rolling-circle mechanism (replication method circling the RNA genome repeatedly to make new copies), and catalytic RNA plays a central role. Every satellite RNA known to rely on a catalytic RNA in rolling-circle replication makes use of hammerhead ribozymes, and three of them also carry hairpin ribozymes in their minus-polarity strand.[8](https://academic.oup.com/nar/article/47/18/9480/5557732) The hammerhead ribozyme, the first structural class of self-cleaving ribozyme discovered, was found in the minus strand of the satellite RNA of tobacco ringspot virus after work on satellite RNA replication.[8](https://academic.oup.com/nar/article/47/18/9480/5557732) The circular form of the RNA affords protection against exonucleases and ensures full-genome replication without initiation or termination tags, a mechanism potentially relevant to primitive RNA self-replicating systems.[1](https://www.tandfonline.com/doi/pdf/10.4161/rna.8.2.14238?needAccess=true)
## Distinction from viroids and satellite viruses
The categories overlap and the terminology has shifted with use. Viroids are plant pathogens consisting of a circular RNA of about 200 to 400 nucleotides that codes for no proteins and shows a complex two-dimensional structure; they replicate by cellular polymerases in a rolling-circle mechanism.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC7123905/) Virusoids share the circular, highly base-paired, viroid-like structure but differ from viroids in requiring a helper virus.[9](https://ndl.ethernet.edu.et/bitstream/123456789/15712/1/9132.pdf) Satellite viruses, in the strict sense, are particles with their own capsid protein, whereas virusoids are nucleic-acid satellites packaged by the helper virus.[7](https://pmc.ncbi.nlm.nih.gov/articles/PMC7173592/)
## Examples
Virusoids are usually found together with plant viruses.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC7123905/) The sobemovirus-associated satellite RNAs for which the term was coined carry the circular form predominantly in particles.[1](https://www.tandfonline.com/doi/pdf/10.4161/rna.8.2.14238?needAccess=true) In medical virology, hepatitis D virus (HDV) is the prominent satellite-like agent: it can proliferate only when the cell is simultaneously infected with hepatitis B virus, and it is described as a virusoid in general virology texts.[2](https://pmc.ncbi.nlm.nih.gov/articles/PMC7123905/) HDV increases the fatality of hepatitis and is the object of intensive therapeutic development.[3](https://pmc.ncbi.nlm.nih.gov/articles/PMC10975798/) Its RNA replicates through a symmetric rolling-circle mechanism catalyzed by host enzymes, with co-transcriptional self-cleavage by HDV ribozymes.[8](https://academic.oup.com/nar/article/47/18/9480/5557732)
## Evolutionary significance
Because viroids and virusoids are small circular RNAs whose replication can involve RNA catalysis and direct interaction with host factors, they have been proposed as relics of the [RNA world](https://www.edgepedia.org/rna-world), the era that preceded the current world based on DNA and proteins, and as molecular fossils of that era.[6](https://www.scielo.br/j/fb/a/tbP38Cwhvvd4yBQqKRctGQJ/?format=html&ilang=pt_BR) The circular structure that protects these RNAs and permits full-genome replication without initiation or termination tags is cited as a mechanism potentially relevant to primitive RNA self-replicating systems.[1](https://www.tandfonline.com/doi/pdf/10.4161/rna.8.2.14238?needAccess=true)
## References
1. [The circular structure of viroid, viroid-like satellite and HDV RNAs, RNA journal (Taylor & Francis)](https://www.tandfonline.com/doi/pdf/10.4161/rna.8.2.14238?needAccess=true)
2. [Viruses: Definition, Structure, Classification, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7123905/)
3. [Viroids, Satellite RNAs and Prions: Folding of Nucleic Acids and Misfolding of Proteins, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC10975798/)
4. [Principles of Medical Virology, Harrison's Principles of Internal Medicine, 22nd Edition, AccessPharmacy](https://accesspharmacy.mhmedical.com/content.aspx?bookid=3541§ionid=294615601)
5. [Satellite Virus, an overview, ScienceDirect Topics](https://www.sciencedirect.com/topics/pharmacology-toxicology-and-pharmaceutical-science/satellite-virus)
6. [Viroids and virusoids: relics of the RNA world, Fitopatologia Brasileira](https://www.scielo.br/j/fb/a/tbP38Cwhvvd4yBQqKRctGQJ/?format=html&ilang=pt_BR)
7. [Virus Replication, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC7173592/)
8. [Novel ribozymes: discovery, catalytic mechanisms, and the quest to understand biological function, Nucleic Acids Research](https://academic.oup.com/nar/article/47/18/9480/5557732)
9. [Virology textbook (Oxford), viroids and satellite RNAs chapter](https://ndl.ethernet.edu.et/bitstream/123456789/15712/1/9132.pdf)
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*Topic: Encyclopedia › Life and health › Microorganisms and fungi › Viruses and acellular agents › Viroids, satellites, and prions › Subviral agents overview*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 21, 2026 · Edited: Sep 30, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license