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Subviral
Agents

Viroid
 Satellite
 Prion


June
9‐11,
2010


A
variety
of
novel
infec7ous
agents
cause
disease
in
plant,
animal
and
 human.

 Several
types
of
nonviral,
subcellular
pathogens
have
disease
–causing‐ poten7al.
These
include
Satellite,
Viroid
and
Prion
 Viroid,
Satellite
and
Prion
are
unusual
infec7ous
agents
characterized
by
having
a
very
small
genome
and
in
the
case
of
prions,
possibly
no
genome
 at
all.
 Because
of
their
simplified
structures
satellite,
prions
and
viroids
are
 some7mes
called
subviral
par7cles
 Conven7onal
strategies
to
combat
virus
infec7ons,
such
as
drug
and
 vaccines,
have
no
effect
on
these
unconven7onal
agents.
 A
beEer
understanding
of
the
biology
of
these
novel
infec7ous
en77es
will
 be
necessary
before
means
of
treatment
of
the
disease
they
cause
will
become
available.




Viroid


Viroid
(Vd)


Viroids
are
common
plant
pathogens
which
are
a
serious
economic
problem
but
 have
recently
been
reported
to
cause
a
human
disease.
 Taxonomy
 Family
Pospiviroidae
 Genus
Pospiviroid;
type
species:
Potato
spindle
tuber
viroid
 Genus
Hostuviroid;
type
species:
Hop
stunt
viroid
 Genus
Cocadviroid;
type
species:
Coconut
cadang‐cadang
viroid
Genus
Apscaviroid;
type
species:
Apple
scar
skin
viroid
 Genus
Coleviroid;
type
species:
Coleus
blumei
viroid
1
 Family
Avsunviroidae
 Genus
Avsunviroid;
type
species:
Avocado
sunblotch
viroid
 Genus
Pelamoviroid;
type
species:
Peach
latent
mosaic
viroid


Discovery
of
Viroids

Potato
spindle
tuber
and
at
least
15
other
crop
diseases
are
caused
by
viroids,
an
en7ty
that
nobody
had
ever
heard
of
before
1971,
its
official
date
of
 discovery.

 TheodorO.
Diener,
the
Agricultural
Research
Service
plant
pathologist
who
 discovered
the
pathogen,
named
it
the
“viroid,”because
it
is
“like
a
virus.”
 Like
a
virus,
the
viroid
invades
a
cell
and
hijacks
its
reproduc7ve
mechanisms.
It
 forces
the
cell
to
duplicate
the
viroid's
RNA
instead
of
its
own.

 The
difference
between
viroids
and
RNA
viruses
is
that
viroids
have
no
protec7ve
protein
coat.
The
scien7fic
dogma
in
1971
was
that
an
organism
 with
no
protein
wasn't
supposed
to
be
able
to
replicate
itself,
even
with
a
 host
cell's
help.
And
an
en7ty
as
small
as
the
PSTV
(potato
spindle
tuber
 viroid)—130,000
daltons—wasn't
supposed
to
be
able
to
infect
anything,
 even
a
potato.



Potato
spindle
tuber



Potato
spindle
tuber

 Viroids
are
Circular,
Single‐stranded,
Non‐coding
RNAs
(~200‐400
 nt.)
that
are
able
to
infect
certain
plants.
 Under
naGve
condiGons
the
mature
circular,
(+)‐stranded
viroid
has
a
 rod‐like
secondary
structure.
This
structure
makes
it
parGally
resistant
 against
nucleases.
(due
to
non‐encapsidated)
 ReplicaGon
does
not
depend
on
the
presence
of
a
helper
virus
 Because
viroids
do
not
code
for
any
pepGde
or
protein,
they
have
to
u7lize
proteins
of
the
host
for
most
biological
funcGons
like
 replicaGon,
processing
or
transport.

 Three
enzymaGc
acGviGes
are
required
for
viroid
replicaGon,
an
RNA
 polymerase,
an
RNAse
and
an
RNA
ligase.



Pospiviroidae
VS
Avsunviroidae


The
structure
of
a
Pospiviroid


Pospiviroid
enclose
CCR
but
lack
of
Ribozyme
ac7vity

 Avsunviroid
:
lack
a
CCR
but
possess
a
ribozyme
ac7vity
Note.

a
ribozyme
is
a
catalyGc
RNA
molecule,
in
this
case
RNA
cleavage
is
the
ribozyme
acGvity


Addi7onally
it
is
speculated
that
Avsunviroids
may
replicate
in
chloroplasts
 whereas
Pospiviroids
replicate
in
the
nucleus
and
nucleolus.


Avsunviroids
replicate
via
a
Symmetric
rolling
circle
mechanism,
 whereas
Pospiviroids
use
an
Asymmetric
mechanism.



Symmetry
and
Asymmetry
Replica7on
the
+ve
infecGng
circular
RNA
strand
of
a
viroid
serves
as
a
template
to
make
a
large
 linear
mulGmeric
‐ve
strand.RNA
pol
II
is
probably
the
enzyme
which
does
this.

 Pospiviroids
with
an
asymmetric
replica7on
pathway
then
make
+ve
RNA
from
this
 long
linear
molecule.
 
A
host
RNAse
acGvity
cleaves
the
+ve
strand
into
unit
viroid
lengths.
This
molecule
is
...
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