Satellites are subviral agents that lack genes that could encode the enzymes needed for their replication. Thus they depend for their multiplication on the co-infection of a host cell with a helper virus. Satellite nucleic acids have nucleotide sequences substantially distinct from those of the genomes of either their helper virus or their hosts.
There are two major classes of satellites: Satellite viruses and Satellite nucleic acids. Satellite viruses are characterised by appearing as distinct nucleoprotein components in preparations of particles of helper viruses. Satellite viruses have nucleic acid genomes that encode a structural protein that encapsidates the genome. Satellite nucleic acids either encode non-structural proteins, or no proteins at all, and are encapsidated by the coat protein of helper viruses.
A related class of agents is that of RNAs that resemble satellites but which encode a function necessary for the biological success of the associated virus. These may be considered as components that remedy a deficiency in a defective virus. These are denoted as satellite-like RNAs, that is they do not encode a replicase, but are classified as being part of the genome of the virus they assist. Examples are the RNA associated with some umbraviruses, or with Beet necrotic yellow vein virus, that contribute to vector transmissibility.
Satellites are genetically distinct from their helper virus by virtue of having a nucleotide sequence substantially different from that of their helper virus. However, the genomes of some satellites have short sequences, often at the termini, that are the same as those in the genome of the helper. This is presumably linked to the dependence of nucleic acids of both satellite and helper virus on the same viral enzymes for replication. Satellites are thus distinct from defective interfering (DI) particles or DI RNAs because these are derived from their ‘helper’ virus genomes. An example of an intermediate state is the chimeric molecules formed from part of a satellite RNA associated with Turnip crinkle virus and part of a DI RNA formed from the virus genome.
Satellites do not constitute a homogeneous taxonomic group. The descriptions in this section are meant only to provide a classification framework and a nomenclature to assist in the description and identification of satellites. The arrangement adopted is based largely on features of the genetic material of the satellites. The nature of the helper virus and of the helper virus host are important secondary characters.
There appears to be no taxonomic correlation between the viruses that are associated with satellites. Satellitism would appear to have arisen independently a number of times during virus evolution. A further complication is that some viruses are associated with more than one satellite. Satellites can even depend on both a second satellite and a helper virus for multiplication.
Most known satellites are ssRNA satellites, with ssRNA plant viruses as helpers. It can be very difficult to distinguish between satellite and genome RNA (e.g., with the dsRNA satellites of fungus viruses) and it is very likely that other satellites, some with novel combinations of characters, remain to be discovered.
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Satellite viruses
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Satellite nucleic acids
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