*RNA is very small- properties of both ss & ds
RNA very infectious- 10 molec/plant -> DZ!! THE BIG QUESTION: how can something so small do so
much?? structure: PSTV- circular RNA c 359 bases, lots of
H- bonds seq studies- similar to introns of plant genes *Diener hypothesis: viroids & virusoids are
escaped introns or have shared ancestors replication: no viral proteins found in viroid
infected cells no replicase- use host enzymes infectivity: structural form is not critical 1678 bases, larger than plant viroids; similar in
structure replicates by rolling circle differences from plant viroids: not naked- packaged in hepatitis B coat protein
& lipids mRNA is complimentary -> delta agent is
minus-strand pseudovirions- host genomic pieces in viral coat satellite virus- dependent on but unrelated to helper
virus req. help for replication satellite RNA- similar in size to viroids; packaged c
helper virus; strain specific; replicates only c
virus cucumber mosaic v c 4 genomic RNAs +/- passenger
RNA- when present, may affect pathogenesis tobacco ringspot: RNA as enzymes came as big news virusoids- subgroup of satellite RNAs satellite RNA & satellite viruses- commonly very
stable due to intramolecular base pairing prion- proteinaceous infectious
agent coined by Prusiner- 1984 based on path & epidem data: incubation- months to
decades path- nervous system: cytopath of select cells in
brain unique- not virus, virusoids, nor anything else
described as infectious *all attempts so far to isolate nucleic acid from
infected tissue failed scrapie most studied: infectious fraction found c amyloid fibrils (rodlike
particles) however, some feel that it is unlikely this protein is
agent- although found in amyloid fibrils of infected
brain tissue of other spp Dz's, ei. CJD *mRNA not found in spleens, yet spleen tissue shows
high infectivity prion protein closely assoc. c agent- role unk if
truly involved Dx: no biochem or TC assay available transmission: vertical(usual) &
horizontal(common); kuru- 1st human slow Dz noted; Papua New Guinea- Fore'
tribe Creutzfeldt-Jakob Dz: no geographic
restriction disinfection of prions: resistant to: autoclaving formaldehyde phenol gluteraldehyde bleach proteases/nucleases ether nonionic & ionic detergents acetone (so-so)
Viroids:most cause plant Dz's- vary from mild to
lethal
prototype- potato spindle tuber viroid
[PSTV]
PSTV 2ndary structure- rodlike: common to many
viroids
**some introns self-splice- autocatalytic excision yields
circular intron RNA & processed parental RNAtranslation initiation site AUG lacking
replication located in nucleolus for some viroids
studied
Hepatitis delta agent:1st circular ssRNA found infecting nonplants-
quite virulent
superinfection c HBV worse than simultaneous infection
contains homologous regions c plant viroids
some homology c grp 1 intronsrequires helper virus- DNA containing
hepatitis B virus for transmission, NOT replication
appears necessary for host receptor recognition
Other small stuff: Satellite RNA & virusoidsDI particles are parts of genome
E.g.: influenza, polio
E.g.: TMV
E.g.: TNSV c tobacco necrosis v.
E.g.: tobacco ringspot v has varying # of
"passengers"
CARNA 5- (cucumber assoc. RNA) found in some
strains1. dimeric or multimeric form in virion
2. spontaneous degradation to monomer
3. split is autocatalytically ligated to cyclic forms
(this may occur c viroids- noted c introns in
splicing)
structurally like viroids, yet req. helper
for replication
PRIONS: grp of 7-8 Dz's caused by "slow viruses" c no
ID virus-like agentscrapie- sheep: world wide; kuru &
Creutzfeldt-Jakob Dz- human
BSE- "mad cow dz"
thought to be common to atypical slow viruses
Dz's-species specificity
* existence of variants, strains, spp
differences- all point to nucleic acid coded information
very stable- isolated from preserved tissue,
found to be still infectious; no virus particles found
PrP [normal isoform] becomes
PrPSc [abnormal isoform] alternate
folding pattern; scapie PrP "directs" folding of PrP
seen also in noninfectious neuropathies
inoculated mice held 8-10 wks p high dose
exposure
orally demonstrated, other means probable as
well
Mad cow disease [bovine spongioform
encephalopathy, a.k.a. BSE]presenile dementia in middle age; incubation
time unk in natural transmission
Control:
Next week:Expression control & strategies- Read Cann-
on reserve; find appropriate parts of Wagner; review
notes for examples of viruses involved
Updated 1/25/04 by thatcher@sonoma.edu