CSB351Y1 Lecture Notes - Lecture 14: Circular Rna, Snake Venom, Viroid
26 May 2018
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Lecture 14: Viroids, Virusoids and Ribozymes
Identification of Viroids (like virus)
• Potato Spindle Tuber Disease (PSTV) – infectious agent
• Cadang-Cadang Disease – coconut palms destruction
• Viroids are found over many parts of the world infecting large number of plant speices
• No arthropods, seed or soil transmission but BY CONTACT in the field and mechanical transmission in the lab
Viroid
• Infectious agents capable of self replication in plants
• Single stranded, covalently closed, circular RNA
• No ORFs or protein produced by viroids
• More than 70% of sequence is in double stranded form
• Extremely stable – boiling for 10-20mins had little impact on infectivity – serious disease in plants
• Consists of few hundred nucleotides of highly complementary, circular, ssRNA but not encapsidated by coat
protein
• Smallest is Rice Yellow Mottle Virus (RYMV) – 220 nt RNA
• Smallest cirus capable of causing infection on their own - 2000 nt
• Viroid RNA does NOT code for protein
• First viroid identified was PSTVd
Classification of viroids
• Viroids common plant pathogens which are a serious economic problem
• 25 different viroid sequences have been determined and numerous variants identified
- Avsunviroidae (chloroplasts, ribozyme)
- Pospiviroidae (nucleus, cell RNase) – two subgroups
Characertization of Viroids (PSTV)
• Sensitivity to nucleases
- Endonuclease:
◼ PSTV + RNase = infectivity destroyed
◼ PSTV + DNase = no effect, infectivity preserved
- Exonuclease:
◼ Snake venom phosphodiesterase (SVPD)
➔ TMV RNA + SVPD = no infectivity
➔ TMV RNA + phosphatase = infectivity
➔ TMV RNA + phosphatase + SVPD = no infectivity
◼ Potato Spindle Tuber Virus
➔ PSTV + SVPD = infectivity
➔ PSTV + phosphatase = infectivity
➔ PSTV + phosphatase + SVPD = infectivity
• PSTV is a circular RNA molecule
Viroid sensitivity to nucleases **
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Document Summary
Classification of viroids: viroids common plant pathogens which are a serious economic problem, 25 different viroid sequences have been determined and numerous variants identified. Characertization of viroids (pstv: sensitivity to nucleases. Pstv + dnase = no effect, infectivity preserved. Tmv rna + svpd = no infectivity. Tmv rna + phosphatase + svpd = no infectivity. Pstv + phosphatase + svpd = infectivity: pstv is a circular rna molecule. Viroid sensitivity to nucleases *: preparation from healthy plants, preparation from pstvd-infected plants, preparation from pstvd-infected plants (+dnase, preparation from pstvd-infected plants (+rnase) Absence of virus particles they are made of naked rna molecules: electron microscopy no particles (can be visualized by shadow-casting) Phenol, sds, proteases do not affect sedimentation properties of pstv. Viroid host interactions: symptoms are varied as those caused by plant viruses, known to change the expression of a wide range of host proteins.
