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Viroids Plant pathogens composed of infectious RNA .

Viroids Plant pathogens composed of infectious RNA . They cause economically important diseases of coconut, potato, avocado, chrysanthemum and other plants. The infectious agent is a single-stranded circular RNA molecule ( ~ 300 bases long) with no protein coat.

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Viroids Plant pathogens composed of infectious RNA .

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  1. Viroids Plant pathogens composed of infectious RNA. They cause economically important diseases of coconut, potato, avocado, chrysanthemum and other plants. The infectious agent is a single-stranded circular RNA molecule ( ~ 300 bases long) with no protein coat. Viroids also catalyse sequence-specific RNA cleavage reactions.

  2. PSTV potato spindle tuber viroid PSTV RNA Phage T7 genomic DNA 38 kb (control).

  3. Viroids replicate by rolling circle replication. Single viroid RNAs excise themselves from a concatameric precursor.

  4. N N N N cleavage site NNNN NNNN NNNN NNNN A region of viroid RNA (~54 bases) forms a ‘hammerhead’ structurethat has catalytic activity. STEM 3 STEM2 STEM 1

  5. Bound metal ion stabilises negative charge. A proton is extracted from the 2’ hydroxyl of a ribose. This attacks the adjacent phosphodiester bond. Cleavage generates a 2’ – 3’ cyclic phosphodiester and a 5’ OH.

  6. Viroid ribozymes A viroid-derived 19-base RNA can cleave multiple complementary RNAs. Substrate strand Enzyme strand Viroid ribozymes can be designed to cleave specific RNAs in cells, e. g. viral RNAs.

  7. RNA editing - changes the information content of mRNA. - only happens with a few genes. There are 2 main types of editing: 1. chemical interconversion of bases. 2. insertion or deletion of bases.

  8. Cytosine can be converted to uridine.

  9. O N H O H 2 H N 2 N H O N N 2 H N N H N N N N N N N R R R + N H 3 Adenine can be converted to inosine. Inosine is read as guanosine during translation. Adenosine Inosine

  10. Apolipoprotein B mRNA editing ApoB 100 (512 kDa) - transports endogenously synthesised lipids. - made in liver cells ApoB 48 (240 kDa) - transports dietary fats. - made in intestinal cells

  11. liver cells intestinal cells CAA UAA In intestinal cells, editing converts the CAA codon for GLN 2153 to a UAA stop codon. Lipoprotein assembly domain. LDL receptor binding domain

  12. In glutamate receptor proteins in rat brain, A  I editing converts a GLN codon to an ARG codon. Editing enzymes are A or C deaminases that recognise specific sequences or 2y structure in the target RNA.

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