 Satellite viruses are viruses which have small
RNA genome and a protein coat.
 Defective viruses can multiply only by
association with a helper virus which
complements the defective gene.
 In satellite viruses, the satellite nucleic acid
codes for its own coat protein.
 They usually act as parasites of associated
helper viruses.

 Satellite viruses have a single-stranded RNA of
800 to 1200 nucleotides.
 They are isometric, 17-nm diameter particles,
built of 60 protein subunits,that differs from
their HV.
 In addition to the open reading frame (ORF)
encoding their coat protein, some satellite
viruses contain further ORFs.

There are two subgroups in this type
subgroup 1- Chronic bee-paralysis virus-
associated satellite virus-Chronic bee-paralysis
satellite virus
Subgroup 2-Tobacco necrosis satellite viruses,
 Maize white-line mosaic satellite virus ,
 Panicum mosaic satellite virus ,
 Tobacco mosaic satellite virus ,
 Tobacco necrosis satellite virus .

 Helper viruses
Aureusvirus, Panicovirus, Tobamovirus, and
Necrovirus.
 These satellite viruses share no sequence
similarities with each other.
 Satellite tobacco mosaic virus (STMV) has a
rod-shaped virus as the helper.

 The RNA of Satellite viruses found associated
with chronic bee-paralysis virus (CPV) consists
of three species, about 1.1 kb.
 which can be encapsidated in 17-nm isometric
particles built of coat proteins encoded by the
satellite or by CPV.
 The satellite interferes with CPV replication.

 The STNV subgroup has 1000 to 1200
nucleotide RNA.
 It has no methylated cap structure or genome-
linked protein at its 5′-end.
 Unlike most plant virus RNAs, it has a
phosphorylated 5′-terminus.
 The particles of STNV may also contain a
noncoding satellite RNA of about 620
nucleotides.
 This depends on TNV for its replication and on
STNV for its encapsidation.

 Satellite RNA are small linear or circular RNA
found inside the virions of certain
multicomponent virus.
 They depend on cognate helper viruses for
replication, encapsidation, movement, and
transmission.
 Satellite RNAs range in size from 200
nucleotides (nt) to approximately 1,500 nt.

 Certain satRNAs code for nonstructural
proteins, but most satRNAs do not encode any
functional protein products.
 Therefore exert their biological functions
through direct RNA interactions.

Subgroup 1: Large satellite RNAs
 Arabis mosaic virus large satellite RNA
 Bamboo mosaic virus satellite RNA
 Chicory yellow mottle virus large satellite RNA
 Grapevine Bulgarian latent virus satellite RNA
 Grapevine fanleaf virus satellite RNA
 Myrobalan latent ringspot virus satellite RNA
 Tomato black ring virus satellite RNA
 Beet ringspot virus satellite RNA

 They contains large satRNAs of about 0.7 to 1.5
kb that encode at least one nonstructural
protein.
 Hence referred to as messenger-type satRNAs.
 They direct protein synthesis both in vitro and in
vivo from a single ORF.

 Most of the Btype satellites are found associated
with nepoviruses, and one has been found associated
with the potexvirus(BaMV).
 Large satellite RNAs, B-type satellites are
dependent upon the helper virus for replication and
encapsidation.

Subgroup 2: Small linear satellite RNAs
 Cucumber mosaic virus satellite RNA
 Cymbidium ringspot virus satellite RNA
 Pea enation mosaic virus satellite RNA
 Groundnut rosette virus satellite RNA
 Panicum mosaic virus small satellite RNA
 Peanut stunt virus satellite RNA
 Turnip crinkle virus satellite RNA
 Tomato bushy stunt virus satellite RNA, B10
 Tomato bushy stunt virus satellite RNA, B1

 They are small, linear RNAs of less than 800
nucleotides and with no circular forms that do
not exhibit any biologically significant
messenger activity.
 Most satellite RNAs in this subgroup modify
the symptoms induced by the HV.
 The most frequent modification is symptom
attenuation, but modulation to more severe
symptoms (eg intense chlorosis or necrosis)
also occurs.

 Subgroup 3: Circular satellite RNAs, or "virusoids"
 Arabis mosaic virus small satellite RNA
 Cereal yellow dwarf virus-RPV satellite RNA
 Chicory yellow mottle virus satellite RNA
 Lucerne transient streak virus satellite RNA
 Solanum nodiflorum mottle virus satellite RNA
 Subterranean clover mottle virus satellite RNA
 Tobacco ringspot virus satellite RNA

 They have small (shorter than 400 nt), circular
RNA genome without biologically significant
messenger activity.
 These satellite RNAs occur as both circular and
linear molecules.
 The HV may encapsidate circular forms (for
sobemoviruses) or linear forms (for
nepoviruses and luteoviruses).
 Small circular single-stranded RNA satellites
interfere with the accumulation and symptoms
induced by the HV.

 Modulate − attenuate or exacerbate − the
symptoms caused by their cognate helper
viruses
 Do not encode their own RNA-dependent RNA
polymerases(RdRps) for their own replication.
 Replication machineries are similar to helper
viruses.
 Study of the replication mechanisms of their
cognate helper viruses.

 Reduce − the accumulation of their cognate
helper viral RNAs.
 Molecular parasites of the helper viruses.
 They can accumulate to high levels in host
plants and thus in some cases can be developed
into high-level expression vectors for foreign
genes.
 Good biological system for study of molecular
biology of the viruses.

 Development of satRNA-based vector systems
for the expression of foreign genes in plants.
 In comparison with plasmids and viral
Vectors it has advantages:
 i) ease of manipulation,
 ii) high in vivo stability,
 iii) high expression level.

 Because of the relatively small size they are
simpler systems to use for cloning, sequencing,
genetic modification, and regular maintenance.
 Most satRNAs are highly structured and thus
significantly more resistant to degradation by
nucleases in vivo than are other viral RNA-based
vector systems. Eg satCMVs
 Most satRNAs and satellite viruses can accumulate
to high levels in host plants, which leads to the
increase in level of proteins translated from the
messenger-type RNAs of the satellites.