Satellite viruses, satellite rna and viroid's

Satellite viruses, satellite rna and viroid's

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  Satellite viruses, Satellite RNAand Viroid's N. H. SHANKAR REDDY Ph.D., Plant Pathology Annamalai University
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  Satellite viruses  The1st satellite virus to be reported is Tobacco necrosis satellite virus (TNV) associated with Tobacco necrosis virus by Kassanis  Satellites viruses are sub-viral agents composed of nucleic acids enclosed by protein coat, due to the lack of necessary genes for replication, they depend on helper virus (another virus) for replication, multiplication and co-infection.  Satellite genomes may be single-stranded RNA or DNA or circular RNA, and are replicated by enzymes provided by the helper virus  Eg – Tobacco necrosis satellite virus
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  • Satellite tobacconecrosis virus (STNV) was the first satellite virus to be recognised, the satellite virus was used to denote to 17 nm isometric particle associated with the 30 nm Tobacco necrosis virus (TNV) isometric particle. Properties  Nucleic acid of small size (0.2-1.5 kbp), share no sequence homology with helper virus for replication.  The nucleotide sequences do not share any homology with that of helper virus.  They are dependent on helper virus for replication.  They code their own coat protein.  Interferes with replication of helper virus.  Satellite RNAs may increase or decrease the severity of viral infection.
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  Satellite RNA  SatelliteRNAs are small, linear or circular RNA strand found in the virions of certain multicomponent viruses.  They do not encode their own coat protein and are dependent upon a helper virus for both replication and encapsidation.  These are 3 types of satellite RNAs, i.e. 1. Large satellite RNAs (B-type), 2. Small linear satellite RNAs (C-type), 3. Small circular satellite RNAs (D-type).
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  Applications of satelliteRNA • Mild strains of helper virus containing satellite RNA to field crops can protect plants from more virulent strains • Producing transgenic plants that expresses the sat RNA sequences • Development of sat RNA based vector system for the expression of foreign genes of plants Advantages of satRNA 1. Ease of manipulation 2. High in vivo stability 3. High expression level
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  Viroid's • They werediscovered by Diener in 1971. • Viroids are RNA molecules, about 246-401nt length; circular having secondary structures. • They do not encode any protein sequences. • Replicate independently using host polymerase • The well studied viroids are potato spindle tuber viroid, Avocado sun blotch viroid, Hop stunt viroid, etc. • The names of the viroids are abbreviated with a “d” added after the disease name.
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  Symptoms • Viroids caninfect both monocots and dicots • The symptoms are more or less similar to those caused by viruses like stunting, mottling, leaf distortion. Sometimes they remain symptomless like Hop latent viroid (HLVd). Inside the plant cells, plasma membrane shows corrugation, thickness of cell wall abnormalities in chloroplast
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  Properties • Viroids sizerange from 246-401nt. • They are circular RNA molecule. While it is rod like in viroids belonging to family Pospiviroidae, the molecules like a branched structure in viroids belonging to Avsunviroidae. The circular molecules have lot of complementary regions giving rise to secondary structures. • On the basis of sequence analysis and secondary predictions following domains are recognized in Potato spindle tuber viroid PSTVd.
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   So far40 viroid diseases have been reported to be caused plant diseases in the world.  The viroid moves from cell to cell through ‘Plasmodesmata’ and ‘cell division’  The replication mechanism of viroid is ‘rolling circle mechanism’  Enzyme used for replication of viroids are ‘RNA polymerase II’  Symptoms range from no symptoms to severe developmental disorders, such as leaf chlorosis, necrosis, stunting, flowering alterations and fruit and seed deformations • Eg – Potato spindle tuber viroid • Coconut cadang cadang viroid
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  Video available atyou tube Channel – Geeky Researcher