Section 5 Flashcards
Plant viruses have the longest history in virology because
first virus discovered is the TMV (tobacco mosaic virus)
Most plant viruses are ______ genomed and there is no normal dsDNA viruses
RNA
plant viruses have diff particle morphologies but high occurrence of _________ structures
and some are related to animal or protist viruses
filamentous
Plant virus common characteristics:
1- most plant virus capsids are made of single protien
2- most are icosohedral
3- most have evolved to get their lipid layer from the cells membrane
4- most geminate making “pairs” of two capsids that come together
Some plant viruses are shaped like a bacillus (rod) called ________ that are helical and filamentous
bacilliform
TMV characteristics
1- perfectly regular helical shape
2- showed that RNA encodes protein
3- good model virus
4- very stable
5- replicates to high density and easy to purify a lot
6- has ONLY two components: RNA genome and capsid protien
packaging of TMV does not start at the end but rather, the RNA has a specific sequence that makes ________ where it goes in first
hairpins
TMV packaging stages
1- RNA specific structure forms hairpin
2- hairpin goes in and forms into a helical structure as protein (capsid) wraps around the outside
3- RNA is fed through the bottom of the capsid as it assembles toward the 5’ end
TMV will spontaneously assemble in vitro when the RNA and capsid protein are mixed together meaning no ____ are required
cofactors
TMV is a great example of efficient use of limited genome size because it encodes _____ different proteins that are
4 different proteins:
2- versions of the non-structural replication protein
- short version ORF1 is a helices required for efficient replication
- long version ORF2 has RdRp region for RNA synthesis**
** (made in smaller amounts cuz ribosome passes through stop codon after ORF1)
the 4 protiens are:
- 2- versions of the non-structural replication protein
and
- 2 protiens translated from subgenomic RNAs
2- versions of the non-structural replication protein are:
(for making -ve sense RNA)
(don’t go into cell)
- short version ORF1 is a helices required for efficient replication
- long version ORF2 has RdRp region for RNA synthesis**
** (made in smaller amounts cuz ribosome passes through stop codon after ORF1)
- 2 protiens translated from subgenomic RNAs means:
this means translation of ORF1-2 from the genomic RNA
needs to happen after infection, and then the RdRp can make
a –ve sense version of the RNA and the sub-genomic RNAs
can be transcribed from that -ve sense copy by RdRp
individual promoters before those ORFs for their transcription
- 2 protiens translated from subgenomic RNAs are
CP = capsid protein, MP = movement protein
How do viruses move within plants?
(not like animals where they spread within the host as assembled particles)
initial local movements between
neighboring cells (without even making complete virions) by MP
followed later by systemic transmission through the plants’ circulatory system (phloem)
what does MP “movement protein” do?
MP = movement protein: allows infection of neighboring
cells by transporting the viral RNA between cells
what happens if virus that does have ORF for CP gets injected?
can spread to neighbouring cells within
the plant but there will not be a systemic infection
viral RNA moves through plant cell structures called __________ (with the requirement of MP)
plasmodesmata
neither the MP nor viral RNA can pass through
plasmodesmata on their own but when ________, they get through
together
MP binds to the viral ssRNA in
_______ regions to move them between plasmodesmata
non-folded
some spherical viruses move between
cells as complete __________
where MP forms a ______ through the
plasmodesmata
nucleocapsids
tube
Cucumber mosiac virus fun facts (2):
- largest host range of any known virus (infects >1200 species of plants)
- infects crops worldwide and is a problem for agriculture
why is cucumber mosaic virus a good model virus
- can infect cells by injecting RNA (without having to make virus particles)
- high yields of particles
- infects plant cell protoplasts (plant cell cultures, don’t need whole plant)
Cucumber mosiac virus genome is
+ve ssRNA segmented genome
RNAs 1, 2, ,3 required for infection
Cucumber mosiac virus also makes
also makes subgenomc RNAs during infection for protein synthesis
Cucumber mosiac virus:
RNAs 1 and 2 are _____ ______
whereas
RNAs 3 and 4 are also _____ _______
(RNA 4 can be made, doesn’t need to come in virion “sub-genomic”)
packaged together
Cucumber mosiac virus needs 3 visions to be transmitted for infection making it a ___-______ ____
multi-partite virus (segmented genome)
cellular enzymes think CMV RNA is tRNA and add an amino acid to it because
3’ ends of CMV fold into tRNA like pseudoknot
CMV folding into tRNA like pseudoknot to trick the cell for many reasons (3):
- protect RNA from being degraded by nucleases
2- increase translation of RNA
3- help avoid RNA replication on RNAs that are being translated
CMV encodes ___ proteins
FIVE
the five CMV protiens are
2 replicase
1 movement within plant and immunity against host defences
1- movement
1 capsid
where does CMV RNA replication occur?
in membrane vesicles attached on the inside of cells vacuole (gas chamber)
(membrane vesicles made by viral proteins and they bring the RNA template there)
How does CMV RNA replication happen in membrane vesicle
replicase interacts with sequences in 5’ and 3’ region of the RNA to initiate replication (also requires host protiens)
WHY does CMV RNA replication occur in membrane vesicles
to keep viral RNA out of cytoplasm and away from anti dsRNA defense
Turnip Yellow Mosaic virus also uses intracellular vesicles to replicate, the vesicle is
outer membrane of chloroplasts
TMV makes vesicles in
endoplasmic reticulum
replication in vesicles is an old strategy as it occurs in primitive cells (ancestors of plants) such as ______ that replicates in vesicles around edges of ER
HaRNAV (relative of polio) infects unicellular photosynthetic protists
_________ forms harmful algal blooms and releases a toxin that kills fish
HaRNAV
Geminiviruses (NEW!) increase packaging size of their simple visions because they have
2 icosahedrons fused together
Geminiviruses (due to their 2 fused icosahedrons)
can package -3000 bases of circular ssDNA and increase the amount of nucleic acid they can package (some are made of 2 pieces of DNA each in separate capsids and both need to be transmitted for infection)
some Geminiviruses are made of 2 pieces of DNA each in separate capsids and both need to be transmitted for infection making them ALSO _____-______
multi-partite
Geminiviruses do not encode their own _____ so so are
dependent on the host enzymes
DNA polymerase for synthesis of the
dsDNA version from ss genome that enters the cell from virion
polymerases
once Geminiviruses use the cells DNA pol to make dsDNA from the ssDNA that enters the cell, this dsDNA then becomes: (2 things)
1- template for transcription (by host
RNA pol)
2- replication of more ssDNA genome copies for packaging into new virions
Geminiviruses have ___________ _____
that go in two directions away
from the intergenic region (IR, top)
overlapping genes
________ _______ (IR) contains (no protein coding) important sequences for replication and gene expression
intergenic region
commonalities of geminivirus with TMV and
CMV, even though it is a ssDNA
virus as opposed to RNA
MP
CP
and Replication protein(s) (don’t do the replication but make it happen)
Key viral protein for geminivirus is _____ that brings cells host DNA pol to perform rolling circular replication
Rep
after Rep is done, it displaces the __ _____ _______ circles that are packaged
+ve ssDNA
Rep binds to a specific sequence in ____ near the stem loop structure and it makes a _____ in the loop portion making the 3’ end needed for DNA synthesis
IR
cut
IR also has promotors for transcription called _________ boxes on both sides to allow gene expression in two directions
TATAA
after completion of a replication cycle, +ve sense DNA circle (template) gets sent back (released)
and new stand gets ______ by Rep to make it into a closed circle (can be packaged or used to make more DNA)
ligated
________ have 10 individual ssDNA circles that make up the genome and package each into its own icosahedral capsid
nanoviruses
plant cells have thick walls which makes it difficult for a virus to get through so they use _____ _______ to break through the plant cell wall and allow transmission
other organisms (ex. insects that feed on plants, ex. worms)
The transmission insects for each viruses are very ____ and _____
specific and limited (ones that feed on that plant)
transmission of plants can also occur vertically through:
seed via infected pollen or ovule
Types of insect transmitted plant viruses
propagative
non-circulative
circulative
Some plant viruses replicate in plant AND in transmission vector species called __________
propagative
Others just stick to the mouth part of the insect and get transmitted to the next plant it eats called ____-______
non-circulative
finally, some intermediate ones move within the insect and pass through cells and tissues but don’t replicate in insect called _________
circulative
Insect transmitted viruses need to be able to interact with _____ ______ cells AS WELL AS the _______ that will carry them by having receptors for that vector on their capsid
plant host
vector
Cucumber necrosis virus (CNV) is transmitted between plants by _____
Fungus
sub viral pathogenic agents common in plants are (2)
satellites
viroids
satellites
- require helper virus for replication (but they are not required for the replication of the helper virus and they don’t share sequence homology with it)
- do not encode their own protiens for replication (in plants)
satellite viruses
encode their own capsid protein that packages the genome but need help for replication
satellite nucleic acids
packaged in helper protein capsid
satellite are _____ of helper
parasites
satellites effect the diseases caused by helper sometimes __________ it and sometimes ________ it
attenuating it
making it more severe
satalties are not a ___________ taxonomic group
they have dsRNA ssDNA and circular ssRNA
homogenous
satellite tobacco mosaic virus (STMV) has ________ genome and and has 3’ untranslated region UTR
similar to that of TMV so that when TMV replicates it replicates the satellite too by accident
ssRNA
UTR
STMV encodes 2 proteins:
1- unknown function
2- capsid
STMV causes _________ disease symptoms of TMV
enhanced
there are satellites that don’t encode proteins but compete w viral RNA for replicase enzyme and packaging called _________
non-messenger satellite RNAs
non-messenger satellite RNAs example
folds into structure that is recognizable by CMV replicase
satelite cucumber mosaic virus
Viriods:
- only exist as generic material (not packaged inside capsids)
- exist as ssRNA molecules
- get transmitted mechanically (attached through something and penetrate the cell wall) through machines etc
viroids are _______ but fold to look like double stranded RNA circle (rod shape)
ssRNA
you can tell viroids by their sequences
beneficial plant viruses (2)
- endogenous pararetroviruses: (package DNA) and protect against infection by exogenous pararetroviruses
- plants infected with some viruses are more resistant to cold temps
- virologists found a fungal virus that is beneficial to plants (grass can survive over 50 deg if it has been infected by fungal virus)
