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Learn the comparison between virusoids and satellites.
In 1981 for the first time J.W. Randles and coworkers discovered the virusoids. These are the small circular RNAs which are similar to viroids but they are always linked with larger molecules of the viral RNA. Robertson (1983) have reported that virusoids are essential for replication of the large RNA and, therefore, form a part of the viral genome.
Branch and Robertson (1984) found that virusoids form structures in the infected cells which suggest that their replication cycles resemble with those Helper virus of the PSTV and several other viroids (e.g. velvet tobacco mosaic virus). The other virusoids like a small satellite (extra RNA molecule associated with virus) is capable to replicate in virus infected host cell.
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Virusoids belong to a large, group of infectious agents called satellite RNAs. They are found in bacteria, plants, fungi, invertebrates and vertebrates. They are satellite, viroid-like molecules but larger than viroids (e.g. about 1000 nucleotides). Satellite viruses encode capsid proteins but depend on a helper virus for replication; therefore, they are called ‘satellite’.
They are packaged as passengers into the virus capsids. They do not interfere with the replication of their helper virus. Therefore, they are differentiated from defective interfering particles that are associated with many viral infections. Five virusoid RNA genomes are 220-338 nucleotides long, circular, single stranded and possess a ribozyme activity.
They replicate in the cytoplasm using an RNA-dependent RNA polymerase. This enzymatic activity is common in plants but not found in animal cells. This helper virus also encapsulates them e.g. subterranean clover mottle virus satellite RNA has a helper Sobemovirus (Fig. 16.26).
Some examples of virusoids are: barley yellow dwarf virus satellite RNA and a helper Luleovirm: tobacco ring spot virus satellite RNA and a helper Nepovirus. These agents modify the symptoms of infection by their helper virus They can be spread by vegetative propagation, within seeds or by direct inoculation either by insects or man.
