Transcript Plant Viruses

VIRUS PROPERTIES
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Infectious – must be transmissible horizontally
Intracellular – require living cells
RNA or DNA genome, not both*
Most all have protein coat*
May of may not have lipid envelope
May have broad or narrow host range
Replication involves eclipse (breaking apart of
virus particles) and reassembly
• Use host factors for to complete replication cycle
SOME CONSEQUENCES AND
EFFECTS OF VIRUS INFECTION
• Like other life forms, viruses promote the
propagation of their own kind
• Like other life forms, viruses evolve in
response to selection pressure
• Viruses are major factors in promoting the
evolution of higher organisms
• Viruses help control populations of their hosts,
including humans
Virus-like agents classified
and studied with viruses
• Viroids
– No coat protein, no coding capacity
• Prions
– No nucleic acid (?)
• Retrotransposons
– No infectivity (?)
Host properties influence the virus
types found in that host group
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Vertebrates have broad range of viruses
Plants have mostly small RNA viruses
Fungi have mostly dsRNA viruses
Single-celled organisms have mostly
large dsDNA viruses
Viruses are
submicroscopic
intracellular
pathogens
Sizes of microscopic and submicroscopic biological entities
and their ability to be examined using various technologies
Flint et al., 2004
Principles of Virology
Fig. 1.8
Viruses may be simple or complex
• Genome sizes 0.3 - 1200 kb; average
genome sizes vary with host organism types
• Isometric particle sizes vary from ~24 nm to
~400 nm diameter
• May have single-stranded (ss) or doublestranded (ds) RNA or DNA genome
• If ssRNA, may be + or – sense
• May have one or many proteins in particles
• May or may not have lipid envelope
Types of viral genomes
• double-stranded (ds) DNA
– Rarely segmented
– Often large
• single-stranded (ss) DNA
– Rarely large
– Less common than dsDNA
• ssRNA, negative sense
– Often found in viruses with broad host ranges
• ssRNA, positive sense
– Most common overall
• dsRNA
– Often segmented
– Particle structure often critical
Composition of viruses infecting
different hosts
• No “rules” about virus families that may or may not
be present in a given kingdom
• Some types of viruses are found more commonly
in some kingdoms than in others
– Many plant viruses contain ssRNA genomes
– Many fungal viruses contain dsRNA genomes
– Many bacterial viruses contain dsDNA genomes
• Host properties determine the types of viruses that
tend to be found in members of a biological
“kingdom”
Virus types by nucleic acid composition
DNA
ss
RNA
ds
ss
ds
Families
Species
env naked env naked env naked env naked
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100 200 300 200 600
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Host type
Vertebrate
Invertebrate
Plant
Fungus
Bacteria
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• True Fungi
Overview of
Virus Properties
– RNA – 2.5-28 kb
– DNA – none
– Enveloped ones have no capsid
– Little genome complexity
– Little morphological complexity
– Some divided genomes
• Prokaryote
– RNA – 5-8 kb
– DNA – 10-200 kb
– Few enveloped
– Range of complexity
– Range of morphologies
– Few divided genomes
• Animal
– RNA – 5-30 kb
– DNA: 5-350 kb
– Many enveloped
– Range of complexity
– Range of morphologies
– Some divided genomes
• Plant
– RNA – 0.3-28 kb
– DNA – 3-10 kb
– Few enveloped
– Little genome complexity
– Little morphological complexity
– Many divided genomes
• Lower eukaryote
– RNA – 5-10 kb
– DNA – 180-1200 kb
– Internal envelope
– Range of complexity
– Range of morphologies
– No divided genomes
Tobacco mosaic virus – a typical small RNA virus
• 18X300 nm
• Single 6400 nt RNA
• 2130 copies of single
17 kDa coat protein
• 3 essential genes
• Simple regulatory
elements
Poxvirus – a typical large dsDNA virus
• 120X200 nm
• Single 180 kb DNA
• Complex coat made
up of numerous
proteins
• >100 essential genes
• Complex regulatory
elements
Virus at the edge: Mimivirus
• Mimivirus infects Acantamoeba polyphaga
• 400 nm particle, 1.2 megabase genome, 1262 putative
ORFs represent the largest virus identified yet
• Many genes for normal cellular functions
– central translation functions
• Aminoacyl tRNA synthetases
• Peptide release factor 1
• Translation elongation factor EF-TU
– DNA repair enzymes
– Many polysaccharide synthesis enzymes
• Lineage suggests connection with eukaryotes, not
prokaryotes
Phylogenetic position of Mimivirus
Compared to other similar DNA viruses
Compared to other prokaryotic
and eukaryotic life forms
Raoult et al., Science Express 10/14/04
Mimiviruses (arrowed) can bee seen inside their amoeba host,
Acanthamoeba polyphaga using a light microscope
Raoult et al., Science Express 10/14/04
By transmission electron microscopy,
isolated Mimivirus can be seen as a
large icosahedral virus with fibrilar
protrusions
Mimivirus (green) seen by
fluorescence microscopy in amoeba
Mimivirus in ultrathin section in amoeba
Raoult et al., Science Express 10/14/04
At 1.2 megabases (1.2X106
nucleotides), Mimivirus has the
largest genome of any known
virus, larger than many bacteria
Simple virus replication cycle
1.
Virus enters
host cell
5. New virus released
from host cell
2. RNA released;
translates
3. Replication in
cytoplasm
4. New virus
assembled
From 7th
Report of the
ICTV
(Academic
Press, 2000)
From 7th
Report of the
ICTV
(Academic
Press, 2000)
From 7th
Report of the
ICTV
(Academic
Press, 2000)
From 7th
Report of the
ICTV
(Academic
Press, 2000)
From 7th Report of the ICTV
(Academic Press, 2000)
From 7th Report of the ICTV
(Academic Press, 2000)
From 7th Report of the ICTV
(Academic Press, 2000)
From 7th Report of the ICTV
(Academic Press, 2000)
From 7th Report
of the ICTV
(Academic
Press, 2000)
From 7th Report
of the ICTV
(Academic
Press, 2000)