Transcript Chapter 17

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Chapter 17
The Viruses:
Bacteriophages
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Classification of Bacterial
and Archaeal Viruses
• the International Committee for the Taxonomy
of Viruses (ICTV) standardizes the viral
classification ~5,000 viruses have been
classified, most being viruses of eucaryotes and
bacteria
– ~40 archaeal viruses have been identified; ~ 8 of
these have been assigned to virus taxa
• based on two major criteria
– capsid structure (but now that is being questioned)
– nucleic acid properties
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Major
phage
families
and
genera
Figure 17.1
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Virulent Double-Stranded
DNA Phages
T-even Phages of E. coli
• lytic cycle
– phage life cycle that culminates with
host cell bursting, releasing virions
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The One-Step Growth
Experiment
mix bacterial host and phage

brief incubation
(attachment occurs)

dilute greatly
(released viruses can’t infect new cells)

over time, collect sample and enumerate viruses
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latent period – no
viruses released
from host
no virions –
either free or
within host
rise period –
viruses released
free viruses
Figure 17.2
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Life Cycle of dsDNA T4 Phage
of E. coli
• adsorption to specific receptor site
• penetration of the cell wall
• insertion of the viral nucleic acid into the
host cell
• transcription  early mRNA
• translation of early mRNA resulting in
production of protein factors and
enzymes involved in phage DNA synthesis
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Phage T4 Life Cycle continued
• transcription  late mRNA
• translation of late mRNA resulting in
synthesis of capsid proteins, proteins
required for phage assembly and
proteins required for cell lysis and
phage release
• cell lysis and phage release
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Figure 17.3
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Adsorption and Penetration
• receptor sites
– specific surface structures on host to
which viruses attach
– specific for each virus
– can be proteins, lipopolysaccharides,
techoic acids, etc.
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Figure 17.4
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Figure 17.5
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Synthesis of Phage Nucleic
Acids and Proteins
• most double-stranded DNA viruses
– use their DNA genome as a template
for mRNA synthesis
• the mRNA is translated to produce viral
proteins
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Replication Strategy Used by
Double-Stranded DNA Viruses
Figure 17.6
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Map of the T4 Genome
early
genes
genes with
related
functions
are usually
found
clustered
together
Figure 17.7
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late
genes
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The T4 Genome
• a large proportion of the genome
codes for replication-related
products including
– protein subunits of its replisome
– enzymes needed for DNA synthesis
• some of these enzymes synthesize
hydroxymethylcytosine (HMC), a
modified nucleotide that replaces cytosine
in T4 DNA
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Figure 17.8
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Synthesis of T4 DNA
• contains
hydroxymethylcytosine (HMC)
instead of
cytosine
– synthesized by
two phage
encoded enzymes
• then HMC
glucosylated
Figure 17.9
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HMC glucosylation
• protects phage DNA from host
restriction endonucleases
– enzymes that cleave DNA at specific
sequences
• restriction
– use of restriction endonucleases as a
defense mechanism against viral
infection
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Post synthesis events
• T4 DNA is terminally redundant
– base sequence repeated at both ends
– allows for formation of concatamers
• long strands of DNA consisting of several
units linked together
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Figure 17.10
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Assembly of Phage Particles
• complex self-assembly process
• involves viral proteins as well as
some host cell factors
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Figure 17.11
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Release of Phage Particles
• in T4 - E. coli system, ~150 viral
particles are released
– two proteins are involved in process
• T4 lysozyme attacks the E. coli cell wall
• holin creates holes in the E. coli plasma
membrane
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Figure 17.12
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Figure 17.13
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Reproduction of X174
A + Strand DNA Virus)
by usual
DNA replication
method
by rolling-circle
mechanism
Figure 17.14
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new
virions
released
by lysis
of host
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Single-Stranded DNA Phages
fd – A Filamentous Phage
–
–
–
–
DNA enters F+, HFr or F’ host
replicative form (RF) synthesized
RF serves as template for mRNA synthesis
virus-encoded protein aids in formation of
phage genome by rolling-circle method
– phage and host have symbiotic relationship
in which new virions are continually released
by a secretory process
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Release
of Pf1
phage
Figure 17.15
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Reproduction of RNA Phages
• most are plus strand RNA viruses
– incoming RNA acts as mRNA and
directs the synthesis of phage proteins
• double-stranded RNA viruses such
as 6 have also been discovered
– Phage 6 usual because it is enveloped
• like MS2 and Qb it attaches to the side of
the F pilus but uses an envelope protein
for adsorption
• life cycle of virus also has unusual feature
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Replication of
Plus-Strand
RNA
Bacteriophages
Figure 17.16
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Temperate Bacteriophages
and Lysogeny
• temperate phages have two
reproductive options
– reproduce lytically as virulent phages
do
– remain within host cell without
destroying it
• done by many temperate phages by
integration of their genome with the host
genome in a relationship called lysogeny
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Lysogeny
• prophage
– integrated phage genome
• lysogens (lysogenic bacteria)
– infected bacterial host
– temperate phages
• phages able to establish lysogeny
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Distinctive characteristics of
Lysogenic Bacteria
• they are immune to superinfection
• under appropriate conditions they
will lyse and release phage particles
– this occurs when conditions in the cell
cause the prophage to initiate synthesis
of new phage particles, a process called
induction
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Lysogenic conversion
• change in host phenotype induced by
lysogeny
– e.g., modification of Salmonella
lipopolysaccharide structure
– e.g., production of diphtheria toxin by
Corynebacterium diphtheriae
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Focus on lambda phage
• doublestranded DNA
phage
• linear genome
with cohesive
ends
– circularizes
upon entry
into host
Figure 17.17
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Lambda Phage DNA
• the DNA contains
12 base singlestranded cohesive
ends
• circularization
results from
complementary
base pairing
Figure 17.18
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The Genome of Phage Lambda
(l)
Figure 17.19
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Infection by Lambda Phage
• Two proteins appear after infection
– the lambda repressor
• product of cI gene
• blocks transcription of the cro gene and
other genes required for the lytic cycle
– Cro protein
• product of cro gene
• inhibits transcription of the lambda
repressor gene
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Lambda Repressor Binding
Figure 17.20
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The Choice…
Figure 17.21
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Table 17.1
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If Lambda Repressor Wins
Race with the Cro protein…
• lysogeny is established
• lambda genome is integrated into the
host genome in a reaction catalyzed
by the enzyme, integrase
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Figure 17.22
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Cro Protein
• binds operator sites OR and OL , blocking
transcription from the PR and PL
promoters
• if Cro protein wins race with lambda
repressor it blocks synthesis of lambda
repressor and prevents integration of the
lambda genome into the host
chromosome
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Cro protein
Figure 17.23
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Induction
• triggered by drop in levels of lambda
repressor
– caused by exposure to UV light and
chemicals that cause DNA damage
• DNA damage alters activity of RecA protein
which interacts with lambda repressor, causing
repressor to cleave itself
• excisionase
– binds integrase
– enables integrase to reverse integration
process
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Bacteriophage Genomes
• the complete sequences of >150 tailed
dsDNA bacteriophages have been
determined
• bacteriophage genomes are mosaic in
character
– blocks of related sequences are shared in
different combinations
• this suggests that lateral gene transfer and
nonhomologous recombination have played a
role in phage evolution.
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Figure 17.24
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