Viral Diversity

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Viral Diversity
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Viruses of Prokaryotes

• Most known bacterial viruses are ds DNA
• RNA() Bacteriophages
• Small (25 nm), icosahedral with 180 copies of
  coat protein per virus particle
• Enteric bacteria infected only if they contain a
  conjugative plasmid
• Infect bacteria by attaching to pili, which are
  encoded for by the plasmid

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Multiplication of MS2

• Infects E. coli
• SS RNA () sense
• 3569 nucleotides
• Small genome encodes four proteins
• maturation protein (mature virus particle)
• coat protein
• lysis protein (release of virions)
• Sub-unit of RNA replicase (involved in
  replication)

Figure 16.2
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Viruses of Prokaryotes (Contd.)

• ss DNA Bacteriophages
• Before genome transcribed, complementary strand
  of DNA must be synthesized (ss-ds)
• fX174 (DNA replication)
• M13 helical symmetry used in genetic
  engineering (cloning vector)

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FX174 (Theta)

• Circular genome
• Ss DNA
• Icosahedral virion
• 25 nm diameter with 60 copies of protein in a
  particle
• Infects E. coli
• 5386 nucleotides
• First DNA to be completely sequenced

Figure 16.3a
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FX174

• How does ss DNA replicate?
• Upon entry ssDNA() becomes separated from
  protein coat and is converted to ds DNA
  (replicative form) using host enzymes (primase,
  DNA polymerase, ligase, gryase)

Figure 16.3b
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Figure 16.4

• How are new progeny formed?
• Rolling circle replication
• One strand nicked by protein A. 3-end used to
  prime synthesis of new strand. Ends with ss
  structure. When long enough, protein A cleaves
  and then ligates two ends.

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Viruses of Prokaryotes (Contd.)

• ds DNA Bacteriophages
• (T4
• Lambda
• T 7
• T3
• Mu induces mutations in host cell used in
  genetic engineering to produce wide variety of
  bacterial mutants )

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• T7
• Infect E. coli Shigella
• Icosahedral head and small tail
• Linear ds DNA
• 39,936 nucleotides
• 17 proteins order of genes influences viral
  multiplication
• Overlapping genes

Figure 16.6
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• T7 Replication
• Bidirectional
• Eye Y shapes can be seen using EM

Figure 16.7a
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Viruses of Eukaryotes

• Plant Viruses
• TMV
• ss RNA() genome
• Helical symmetry with 2130 copies of coat protein
• 6395 nucleotides and four proteins
• Infects tomato and tobacco plants
• Expression somewhat more complicated as
  eukaryotes (plants) cannot translate
  polycistronic mRNA (sub-genomics mRNAs are made)
  and each is translated to make one protein
• Chlorella viruses (green algae)
• Paramecium bursaria CV 1 (PBCV1)
• ds DNA genomes
• Large Icosahedral virions

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Viruses of Eukaryotes

• Animal Viruses
• RNA()
• Picornaviridae (polioviruses, rhinoviruses)
• 30 nm in diameter (picosmall)
• ss RNA
• Simple icosahedral structure with 60
  morphological unites per virion, each with 4
  distinct proteins
• For poliovirus, the RNA is monocistronic, but
  codes for all the proteins in a single protein
  called a polyprotein
• Polyprotein later cleaved into individual
  proteins

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Poliovirus
Figure 16.12
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Coronaviruses

• Ss RNA ()
• Larger size than Picornaviruses
• Enveloped club shaped spikes coronacrown
• Cause variety of upper respiratory tract
  infections common cold, SARS

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Viruses of Eukaryotes

• Animal Viruses
• RNA(-)
• Rhabdoviridae (e.g. rabies)
• Enveloped with bullet shaped virus particle (70 x
  175 nm) nucleocapsid is helically symmetrical
• Contains RNA-dependent RNA polymerase
• Orthomyxoviridae (e.g. influenzae)
• Viruses interact bind to mucous surfaces (myxo)
  and transmitted by respiratory routes
• Enveloped with segmented genomes Antigenic
  shift
• Segments recombine result in new forms of
  virus with different surface proteins
• Vaccines do not provide immunity longer than a
  year

Figure 16.15
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Multiplication of RNA(-)
Figure 16.14
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Viruses of Eukaryotes

• ds RNA (Reoviridae)
• Largest group of animal viruses (e.g. Rotavirus)
• Non-enveloped nucleocapsid, 60-80 nm in diameter
• RNA-dependent RNA polymerase within the virion
• Genome contains 10-12 molecules of ds RNA
  difficult to unwind
• Replication in host cytoplasm
• Transcription using the minus strand as a
  template to make mRNA
• ss RNA() act as template for ds RNA progeny

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Viruses of Eukaryotes

• Several families that infect humans
• Polyomaviridae
• some induce tumors in animals (Simian virus 40)
• Papillomaviridae
• Human papilloma virus can cause skin and
  cervical cancers
• Herpesviridae
• cause wide variety of diseases in humans and
  animals including fever blisters, venereal
  herpes, chickenpox, shingles, mononucleosis
• Poxviridae
• Smallpox
• Adenoviridae
• cause mild respiratory infections in humans

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SV 40

• Simple, non-enveloped particle 45 nm in diameter
  with icosahedral head containing 72 protein
  subunits
• Genome contains one molecule ds DNA of 5423 base
  pairs
• Introns excised out of primary mRNA transcript
• Used host DNA polymerase

Figure 16.18
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Figure 9.23
Figure 16.19
All portions of viral genome incorporated into
host cell DNA. Viral genes that encode
transforming information are processed to viral
mRNA particles. Viral mRNA particles are
transported to cytoplasm. Translated to form
transforming proteins which convert normal cells
to cancer cells.
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Herpesviruses

• Herpes simplex I
• enveloped virus is 150 nm in diameter
• icosahedral symmetry of 162 capsomeres of
  distinct proteins
• One large ds DNA molecule of 152,260 nucleotides
• Three classes of mRNA
• Intermediate early Regulatory proteins
• Delayed early DNA replication
• Late Proteins of viral particle

Figure 16.20
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Pox Viruses

• Among the largest and complicated genomes
  (smallpox, cowpox, rabbit myxomatosis virus)
• Replicate in the cytoplasm
• Genomes are comprised of ds DNA, can be as large
  as 185kbp and contain 150-200 genes
• Ends of dsDNA have hairpin like feature

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Adenoviruses

• Icosahedral, ds DNA viruses first isolated form
  the tonsils and adenoid glands of humans
• Genomes are linear, consist of about 36kbp
• DNA has terminal inverted repeats of about
  100-1800 bp in length
• Replication in nucleus

Figure 16.22
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Retroviruses

• Enveloped RNA genome with DNA intermediate
• Reverse transcriptase to make DNA from RNA

Figure 9.24
Other interesting features (1) first known
cancer involvement, (2) AIDS cause, (3)integrated
into host via DNA intermediate as means of
introducing foreign DNA into host (gene therapy)
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Genome Features

• Two identical ss RNA molecules () sense
• Regions gag (internal structure), pol
  (transcriptase), env (envelope) always this order

Figure 9.24
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Figure 9.25
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(No Transcript)
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Retroviridae

• RNA reverse-transcribing viruses Retroviruses
• e.g. HIV-1
• DNA reverse-transcribing viruses Hepadnaviruses
• e.g. HBV
• One of smallest known viruses, but complex life
  cycle
• Replication
• Transcription of RNA polymerase
• Viral polymerase copies this into DNA
• DNA packaged into new particles