A1260288038cDfiw

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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.
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PSTV potato spindle tuber viroid
PSTV RNA
Phage T7 genomic DNA 38 kb (control).
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Viroids replicate by rolling circle replication.
Single viroid RNAs excise themselves from a
concatameric precursor.
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A region of viroid RNA (54 bases) forms a
hammerhead structure that has catalytic
activity.
STEM 3
STEM 1
STEM2
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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.
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Viroid ribozymes can be designed to cleave
specific RNAs in cells, e. g. viral RNAs.
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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.
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Cytosine can be converted to uridine.
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Adenine can be converted to inosine. Inosine is
read as guanosine during translation.
Adenosine
Inosine
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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
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In intestinal cells, editing converts the CAA
codon for GLN 2153 to a UAA stop codon.
Lipoprotein assembly domain.
LDL receptor binding domain
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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.