Properties of Viruses

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Virus properties Virus taxonomy Structure Detec
tion and diagnosis Viral infection cycle Gene
expression Replication Movement
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VIRUSEGeneS!! What they are ??
General Characteristics of Viruses

• Nucleoprotein
• nucleic acid surrounded by a protein coat
• acellular, lacks a metabolism of its own
• Obligate parasites
• can only multiply in living cell
• has the ability to cause disease
• Submicroscopic
• too small to be seen individually with a light
  microscope
• electron microscope (EM)

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Viruses The Ultimate Parasites

• No physiologic activity
• No energy production
• Do not divide
• Do not produce any kind of specialized
  reproductive structures
• Multiply by inducing host cells to make more virus

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• Cause disease by utilizing cellular
  
• substances during multiplication
• Interfering with host metabolism and
  developmental regulation
• Do not consume cells or kill them with toxins

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Viruses The Ultimate Parasites

• Smaller than the smallest prokaryotic cell
• Variable size range
• small--25 nm
• larger--300 nm

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Fig. 3. Classification of plant viruses. From
Van Regenmortel, M.H.V., Fauquet, C.M., Bishop,
D. et al. (1999). Virus Taxonomy Seventh Report
of the International Committee on Taxonomy of
Viruses.
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Positive strand RNA viruses are important
()RNA (-)RNA Retrovirus ssDNA dsRNA dsDNA
Ex Hepatitis A and C Poliovirus
West Nile virus Coronaviruses (SARS)
Tobacco Mosaic Virus Brome Mosaic Virus
From Fields Virology, 4th Edition 2004
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Luteoviruses / Potyviruses
Geminiviruses
Tobacco mosaic virus
Tospoviruses
Rhabdoviruses
The Big Picture Book of Viruses
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Plant Viruses Nucleic Acids
ss RNA viruses () RNA Tobacco mosaic virus
(-) RNA Tomato spotted wilt virus ds RNA
viruses Rice fiji disease virus ds DNA
viruses Cauliflower mosaic virus ss DNA
viruses Tomato yellow leaf curl virus
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What it is a VIRION?

• The Infectious viral Unit
• Monopartite one type of virus particles
• Multicomponent (split genomes)
• bipartite two types of virus particles
• tripartite three types of virus particles
• All must be in plant for disease development
• Different length rods
• Different density isometric particles

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Multipartite/Multiparticulate
BMV
TRV
TMV
AMV
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Multipartite genome

• Brome mosaic virus
• The genome is segmented tripartite, segments are
  distributed among 3 particles

RNA 3
RNA 1
RNA 2
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Template-switching model for viral RNA
recombination
A
B
Parental RNAs
a
b
Polymerase
a
Recombinant RNA
B
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REASSORTMENT OF GENOME SEGMENTS OCCURS BETWEEN
GENOTYPES WITHIN A TOSPOVIRUS SPECIES
Coinoculation

• Reassortants may have altered properties, e.g.
  responses to plant resistance, thrips
  transmission properties.
• Reassortment is one way in which tospovirus
  populations may respond to selection pressure
  (e.g. from the plant or insect host).

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Virus Structure
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Coat Protein

• Provides protective sheathing for NA
• Determines vector transmissibility
• Determines symptoms
• Assembly
• Inclusion bodies

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Rod Shaped and Variations
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Coat protein Composition?

• Protein shell composed of repeating subunits,
  constant for a given virus
• TMV protein subunit 158 amino acids
• Protein subunits arranged in a helix
• Diameter 18 nm
• Central hole diameter 4 nm
• NA packed tightly between the helices
• NA in polyhedral viruses folded in unknown matter
  inside the shell

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Icosahedral structure of viruses in the
Bromoviridae
For these types of icosahedral viruses (T3),
virions consist of 180 coat protein subunits
Speir et al., Structure 363-78 (1995)
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Speir et al., Structure 363-78 (1995)
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Diagnosis and Identification
General field observations Symptoms

• In field symptoms
• Indicator plants
• Chenopodium quinoa, Ch. amaranticolor
• N. glutinosa
• N. benthamiana
• etc, etc, etc, etc
• Virus morphology
• Viral nucleic acid components and unique nucleic
  acids in cells
• Viral Protein characteristics
• ELISA
• Protein physical properties (Molecular
  weight, no. of species etc)
• Cytopathic effects
• Inclusion bodies in infected cells
• Membrane irregularities etc

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Cucumber Mosaic Virus
Three genomic RNAs
Single Coat Protein
Isometric Virions
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Enzyme Linked Immunosorbent Assay ELISA
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Light Microscopic Identification
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