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sat. virus.ppt

  1.  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.
  2.  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.
  3. 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 .
  4.  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.
  5.  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.
  6.  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.
  7.  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.
  8.  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.
  9. 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
  10.  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.
  11.  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.
  12. 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
  13.  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.
  14.  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
  15.  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.
  16.  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.
  17.  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.
  18.  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.
  19.  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.