LECTURE PRESENTATIONS
For BROCK BIOLOGY OF MICROORGANISMS, THIRTEENTH EDITION
Michael T. Madigan, John M. Martinko, David A. Stahl, David P. Clark
Chapter 21
Viral Diversity
Lectures by
John Zamora
Middle Tennessee State University
© 2012 Pearson Education, Inc.
I. Viruses of Bacteria and Archaea
•
•
•
•
•
•
21.1 RNA Bacteriophages
21.2 Single-Stranded DNA Bacteriophages
21.3 Double-Stranded DNA Bacteriophages
21.4 The Transposable Phage Mu
21.5 Viruses of Archaea
21.6 Viral Genomes in Nature
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21.1 RNA Bacteriophages
• Most bacteriophages have double-stranded
DNA genomes, however, many other types
are known
• RNA genomes are the simplest
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21.1 RNA Bacteriophages
• Many RNA bacteriophages contain RNA
genomes of the plus (+) configuration
• RNA viruses of enteric bacteria attach to
bacterial pili (Figure 21.1)
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Figure 21.1
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21.1 RNA Bacteriophages
• Phage MS2
– An example of a bacterial RNA virus that infects
E. coli
– Possesses a small genome that is directly
translated by a combination of host and viral
enzymes (Figure 21.2)
– Possesses overlapping genes
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Figure 21.2
Lysis protein
Maturation
protein
1 130
Coat
Replicase
1308 1335 1724 1761
3395 3569
Genetic map of MS2
Viral genome
(ssRNA, )
Minus strand
synthesis
Genome used
directly as mRNA
(ssRNA, )
Translation
Plus strand
synthesis
RNA
replicase
(ssRNA, )
Assembly
Lysis protein
production
Progeny virions released
Flow of events during viral multiplication
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Viral
proteins
21.2 Single-Stranded DNA Bacteriophages
• Some bacteriophages contain singlestranded DNA genomes of the plus
configuration
• Transcription of the genome is preceded by
synthesis of a complementary strand of DNA
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21.2 Single-Stranded DNA Bacteriophages
• Bacteriophage X174
– Contains a circular single-stranded DNA genome
inside an icosahedral virion (Figure 21.3)
– Very small genome with overlapping genes
– Replication occurs via rolling circle replication
(Figure 21.4)
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Figure 21.3
Origin of genomic
replication
A
A*
H
B
Overlapping
genes
K
C
D
G
E
J
F
A
A*
B
C
D
Replicative form DNA synthesis
Shutoff of host DNA synthesis
Formation of capsid precursors
DNA maturation
Capsid assembly
E
F
G
H
J
K
Host cell lysis
Major capsid protein
Major spike protein
Minor spike protein
DNA packaging protein
Function unknown
Genetic map of 174
Transcription
off of  strand
mRNA ()
ssDNA
Replicative
Replication
form (dsDNA) by rolling
circle
Flow of events during 174 replication
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ssDNA
(viral genome)
Figure 21.4
Gene A protein
Cut site at origin
174 replicative
form
3 end of  strand
Growing point
(3 end of second
progeny strand)
Displaced strand
Growing point
Roll
3 end of second
progeny strand
One progeny virus genome
Roll
One revolution
complete
Cleavage and
ligation by
gene A protein
174 replicative
form ready for new
genome synthesis
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174 genome
(ssDNA)
21.2 Single-Stranded DNA Bacteriophages
• Bacteriophage M13
– Model filamentous bacteriophage
– Used as a cloning and DNA-sequencing vector
in genetic engineering
– Can be released without lysing host via a
process called budding (Figure 21.5)
– Viral infection slows host growth
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Figure 21.5
P3 and P6
P3 and P6
Outer
membrane
P8
P8
Channel
proteins
Cytoplasmic
membrane
P8 in
membrane
Viral genome (ssDNA)
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P7 and P9
21.3 Double-Stranded DNA
Bacteriophages
• Bacteriophage T7
– Infects E. coli
– Virion has an icosahedral head and very short
tail (Figure 21.6)
– Genome always enters host cell in same
orientation
• Order of genes on the T7 chromosome
influences regulation of virus replication
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Figure 21.6
Gene designation
Function
Left end
Early
promoters 0.3
Transcribed
by host RNA
polymerase
Promoter
Promoter
Promoter
Transcribed
by T7 RNA
polymerase
Promoter
Inhibits host restriction
0.7
Protein kinase
1
1.1
T7 RNA polymerase
Unknown
1.3
1.7
2
3
3.5
DNA ligase
Nonessential
Inactivates host RNA polymerase
Endonuclease
Lysozyme
4
Helicase, primase
5
DNA polymerase
6
Exonuclease
7
8
Virion protein
Head protein
9
10
Head assembly protein
Major head protein
11
12
Tail protein
Tail protein
13
14
Virion protein
Head protein
15
Head protein
16
Head protein
17
Tail protein
18
DNA maturation
19
DNA maturation
Origin of DNA replication
Promoter
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Bacteriophage T7
Major properties of T7
Proteins for
DNA replication
and host lysis
1. Replication cycle requires 25 minutes
2. Genome is linear double-stranded DNA
of 39,936 bp
3. T7 encodes all of its own proteins for
DNA replication and transcription
4. Time to complete 100 T7 genome copies
from a single copy: 5 minutes
5. Burst size (Escherichia coli host): about
300 virions/cell
6. Head size, 45 nm
Phage structural
components and
maturation
proteins
7. Forms large plaques
8. T7 promoters are unique and widely
used in biotechnology
21.3 Double-Stranded DNA
Bacteriophages
• Bacteriophage T7 (cont’d)
– DNA replication employs T7 DNA polymerase
and involves terminal repeats and the
formation of concatemers (Figure 21.7)
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Figure 21.7
Terminal
repeat
Origin of
replication
Left end
“Eye” form
“Y” form
Concatemer
Completed strands
DNA
polymerase
Cutting
enzyme
Pairing of unreplicated terminal repeats;
DNA polymerase and ligase activity
DNA
polymerase
Joining of new and old molecules, forming
a concatemer
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Cutting enzyme
(arrows) makes
single-stranded
cuts
DNA polymerase
completes the
single strands
Mature T7
molecule,
with terminal
repeats
21.4 The Transposable Phage Mu
• Bacteriophage Mu
– “Mutator” phage
• Induces mutations in host genome
–
–
–
–
Useful in bacterial genetics
Temperate phage
Replicates by transposition
Large virus with an icosahedral head, helical
tail, and six tail fibers (Figure 21.8)
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Figure 21.8
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21.4 The Transposable Phage Mu
• Bacteriophage Mu (cont’d)
– Invertible G region of genome determines
host range
– Genome is integrated into the host
chromosome via a transposase (Figure 21.9)
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Figure 21.9
Insertion point: region
to be duplicated
Host
DNA
Staggered cuts made by
transposase in host DNA
Positive activator
of late mRNA synthesis
Repressor
Lysis
Integration
replication
Invertible G
segment
(host range)
Head and
tail genes
Conversion to single
strands and insertion of Mu
Variable
end
(host DNA)
Host
DNA
lys
attL
Transposase
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attR
Repair of DNA leads
to formation of five-basepair duplication
21.4 The Transposable Phage Mu
• Bacteriophage Mu (cont’d)
– In both lytic and lysogenic pathways the genome
is replicated as part of a larger DNA molecule
– Lysogenic state requires sufficient amounts of a
repressor protein to prevent transcription of
integrated Mu DNA
– Genome is packaged into the virion with short
(5 bp) sequences of host DNA at either end
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21.5 Viruses of Archaea
• Most viruses that infect Archaea resemble those
that infect enteric bacteria (Figure 21.10)
• Only double-stranded DNA viruses have been
identified so far
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Figure 21.10
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21.6 Viral Genomes in Nature
•
•
•
•
Total prokaryotic cells on Earth = 1030
Total viruses on Earth = 1031
Most of the viruses are bacteriophages
Most of the genetic diversity on Earth is thought
to reside in viruses
• Viral metagenome is the sum total of all viral
genes in an environment
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II. RNA Viruses of Eukaryotes
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•
•
•
•
21.7 Plant RNA Viruses
21.8 Positive-Strand RNA Animal Viruses
21.9 Negative-Strand RNA Animal Viruses
21.10 Double-Stranded RNA Viruses: Reoviruses
21.11 Retroviruses and Hepadnaviruses
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21.7 Plant RNA Viruses
• Most plant viruses are positive-strand RNA
viruses
– Example: tobacco mosaic virus (Figure 21.12)
• Genomes can move within the plant host
through intercellular connections that span
the cell walls
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Figure 21.12
Stop
codon
Cap
MTH
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tRNA-like
structure
RNP
MP
CP
21.8 Positive-Strand RNA Animal Viruses
• Replication of positive-strand RNA viruses
requires a negative-strand RNA intermediate
from which new positive strands are
synthesized
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21.8 Positive-Strand RNA Animal Viruses
• Poliovirus (Figure 21.13a)
– Small virus
– Viral RNA is translated directly, producing a
single long, giant protein (polyprotein) that
undergoes self-cleavage to generate ~20 smaller
proteins necessary for nucleic acid replication
and virus assembly (Figure 21.13b)
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Figure 21.13a
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Replicase-mediated
Figure 21.13b
Synthesis of new plus strands
Poliovirus genome
Synthesis of minus strand
Poly(A)
VPg
Translation
Polyprotein
Proteases
cleave
polyprotein
Structural coat
proteins
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Proteases
RNA replicase
21.8 Positive-Strand RNA Animal Viruses
• Poliovirus (cont’d)
– Host RNA and protein synthesis are inhibited
when poliovirus replication begins
Animation: Replication of Poliovirus
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21.8 Positive-Strand RNA Animal Viruses
• Coronaviruses
– Larger virus
– Cause respiratory infections, including SARS, in
humans and other animals
– Virions (Figure 21.14a)
• Are enveloped
• Contain club-shaped glycoprotein spikes on their
surfaces
• Produces monocistronic mRNA (Figure 21.14b)
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Figure 21.14a
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Figure 21.14b
Infection, ssRNA
genome released
Cap
Replicase
gene
Translation of
replicase gene
Replicase
Synthesis of () strand
Synthesis of
monocistronic mRNAs
Synthesis of
genome copies
Translation to
yield viral proteins
Viral assembly
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21.9 Negative-Strand RNA Animal Viruses
• Negative-strand RNA viruses
– Negative-strand RNAs are complementary to
the mRNA
• They are copied into mRNA by an enzyme
present in the virion
– Only those that infect Eukarya are known
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21.9 Negative-Strand RNA Animal Viruses
• Rhabdoviruses
– Include viruses that cause
• Rabies in animals and humans
• Vesicular stomatitis in cattle, pigs, and horses
– Enveloped viruses
– Virion is bullet-shaped (Figure 21.15)
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Figure 21.15
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21.9 Negative-Strand RNA Animal Viruses
• Rhabdoviruses (cont’d)
– RNA of rhabdoviruses is transcribed in the
host cytoplasm into two distinct classes
(Figure 21.16):
1. Series of mRNAs encoding the structural
genes of the virus
2. Positive-strand RNA that is a copy of the
complete viral genome
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Figure 21.16
Transcription by viral
RNA polymerase
 Strand parental RNA
RNA
polymerase
mRNAs ( sense)
Translation
(using host enzymes)
 Strand RNA
RNA
polymerase
Proteins
Assembly
Envelope
 Strand genomic RNA
Progeny virus
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21.9 Negative-Strand RNA Animal Viruses
• Influenza
– Enveloped, polymorphic virus
– Segmented genome
– Surface proteins interact with host cell surface
• Hemagglutinin causes clumping of red blood cells
• Neuraminidase breaks down sialic acid
component of host cytoplasmic membrane
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Figure 21.17
Neuraminidase
Hemagglutinin
Viral RNA polymerase
RNA endonuclease
Envelope
RNA genome (eight segments)
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21.9 Negative-Strand RNA Animal Viruses
• Processes that help influenza elude the host
immune system
– Antigenic shift
• Portions of the RNA genome from two genetically
distinct strains of virus infecting the same cell are
reassorted
• Generates virions that express a unique set of
surface proteins
– Antigenic drift
• Structure of neuraminidase and hemagglutinin
proteins are subtly altered
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21.10 Double-Stranded RNA Viruses:
Reoviruses
• Reoviruses (Figure 21.18)
– Nonenveloped nucleocapsid with a double
shell of icosahedral symmetry
– Virions contain virus-encoded enzymes
necessary to synthesize mRNA and the new
RNA genomes
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Figure 21.18
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21.10 Double-Stranded RNA Viruses:
Reoviruses
• Reoviruses (cont’d)
– Genome segmented into 10–12 molecules of
linear double-stranded RNA
– Replication occurs exclusively in host
cytoplasm
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21.11 Retroviruses and Hepadnaviruses
• Retroviruses (RNA viruses) and hepadnaviruses
(DNA viruses) use reverse transcriptase for
replication
– Retroviruses
• Enveloped virions that contain two copies of the
RNA genome
• Virion contains several enzymes
– Includes reverse transcriptase used to make DNA
copy of genome (Figure 21.19)
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Figure 21.19
Direct repeats
Retroviral RNA
R
PB
Reverse transcription of RNA into
DNA of  100 nucleotides at the 5
terminus by reverse transcriptase
R
Primer tRNA
Removal of terminally redundant
viral RNA by reverse transcriptase
ribonuclease H activity
New DNA
Transfer of DNA and tRNA to 3 end
New DNA
Continued synthesis leads
to extension of  strand DNA
Primer
New DNA
Ribonuclease H activity removes
all of  strand RNA except small
fragment used as primer
Completion of short segment of  strand
DNA and removal of both primers
Reverse transcriptase transfers to other
strand and completes complementary
() strand DNA synthesis
LTR
LTR
Formation of double-stranded
DNA through activity of
reverse transcriptase
Integration into host chromosomal DNA to form provirus state
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21.11 Retroviruses and Hepadnaviruses
• Retroviruses (cont’d)
– Gene expression and protein processing are
complex (Figure 21.20)
– All retroviruses have the three genes:
• gag encodes several small viral structural
proteins
• pol is translated into a large polyprotein
• The env product is processed into two distinct
envelope proteins
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Figure 21.20
Cap
Processing
of GAG
gag
*
pol
env
GAG
GAG - POL
Processing
of GAG-POL
Capsid
proteins
Protease
Cap
Reverse
transcriptase
env
Deleted
region
ENV
EP1
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POL
EP2
Integrase
21.11 Retroviruses and Hepadnaviruses
• Hepadnaviruses
– Virions small, irregular-shaped particles
(Figure 21.21a)
– Include hepatitis B
– Viral replication occurs through an RNA
intermediate
– Unusual genomes
• Tiny
• Only partially double-stranded (Figure 21.21b)
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Figure 21.21a
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Figure 21.21b
Transcripts
2.4 kb
Viral genome
RNA primer
of  strand
2.1
kb
Protein primer
of  strand
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3.4 kb
0.7 kb
IV. DNA Viruses of Eukaryotes
•
•
•
•
•
21.12
21.13
21.14
21.15
21.16
Plant DNA Viruses
Polyomaviruses: SV40
Herpesviruses
Pox Viruses
Adenoviruses
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21.12 Plant DNA Viruses
• Plant DNA viruses are rare
• However, some unusually large viruses are
known that infect unicellular plants
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21.12 Plant DNA Viruses
• Paramecium bursaria Chlorella virus 1 (PCBV-1)
– Infects the green alga Chlorella
– Large icosahedral virion (Figure 21.22)
– Large, double-stranded DNA genome encoding
several hundred proteins including several
restriction/modification enzyme systems
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Figure 21.22
Capsid
Lipid membrane
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21.13 Polyomaviruses: SV40
• SV40, a polyomavirus
– Induces tumors in animals
– Nonenveloped virion with an icosahedral head
– No enyzmes in the virion; replicates in host
nucleus
– DNA is circular (Figure 21.23)
– Small genome, has overlapping genes
(Figure 21.24)
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Figure 21.23
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Figure 21.24
VP3
intron
Origin of
replication
Small T
intron
Large T intron
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21.13 Polyomaviruses: SV40
• Some polyomaviruses cause cancer
– In permissive host cells, virus infection results
in the formation of new virions and the lysis of
the host cell
– In nonpermissive host cells, the virus DNA
becomes integrated into host DNA (analogous
to a prophage), genetically altering cells in the
process (Figure 21.25)
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Figure 21.25
Tumor virus DNA
Host DNA
Infection
Integration
Viral DNA integrated
into host DNA
Transcription
Tumor virus mRNA
Transport to cytoplasm
and translation
Viral
proteins
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Transformation of
cell to tumor state
21.14 Herpesviruses
• Herpesviruses
– Large group of viruses that cause diseases in
humans and animals
– Able to remain latent for extended periods of
time
– An important group causes clinical forms of
cancer
• Example: Epstein–Barr virus
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21.14 Herpesviruses
• Herpesviruses (cont’d)
– Infection follows attachment of virions to specific
cell receptors
– Three classes of mRNA are produced
(Figure 21.26):
• Immediate early, encodes five regulatory proteins
• Delayed early, encodes DNA replication proteins
• Late, encodes structural proteins of the virus
particle
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Figure 21.26
Viral DNA
mRNA
Immediate
early proteins
Delayed
early proteins
Late proteins
Rolling circle replication
Selfassembly
Viral genomic DNA
Progeny virus
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21.15 Pox Viruses
• Pox viruses
– Among the most complex and largest animal
viruses known (Figure 21.27)
– Replicate in the cytoplasm
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Figure 21.27
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21.16 Adenoviruses
• Adenoviruses
– Major group of icosahedral linear doublestranded DNA viruses
– Cause mild respiratory infections in humans
– Replicate in the nucleus
– Replication requires protein primers and avoids
synthesis of a lagging strand (Figure 21.28)
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Figure 21.28
Adenovirus DNA
Terminal
protein
Plus strand copied
Leading
strand
Direction of
cyclization
Minus strand cyclizes
via inverted terminal
repeats
Minus strand copied
Leading
strand
Completed linear
double strand
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