Download Types of Viral Mutation A. Point Mutations Conditional

GENETIC
VARIATION OF
VIRUSES
Part 2
Lecture 3
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Types of Viral Mutation
A. Point Mutations
Point mutations occur when a single nucleotide is
changed.
 Point mutants with specific characteristics are often
selected in the laboratory and may belong to one of
the following types:

1.
2.
3.
4.
5.
6.
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Conditional-lethal mutants
Plaque-size mutants
Host-range mutants
Drug-resistant mutant
Enzyme-deficient mutants
Hot mutants
Types of Viral Mutation
A. Point Mutations
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Types of Viral Mutation
A. Point Mutations
1.
Conditional-lethal mutants:
◦ These can multiply under some conditions but not
under the conditions that allow the multiplication
of wild viruses. For example:
a) Temperature-sensitive (t.s.):
 Mutants do not replicate well at temperatures between
36oC and 41oC; therefore, they are attenuated strains in
humans.
 Their antigenic make-up is virtually identical to that of
the wild strain, and consequently, these mutants are
often used for immunization purposes.
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Types of Viral Mutation
A. Point Mutations
b) Host-dependent mutants:
 The mutation results in the replacement of an amino
acid codon by a termination (stop) codon.
 Such a mutation is called a “nonsense” mutation.
 Viruses with these mutations will replicate only in host
cells that have mutated tRNA that can recognize the
termination codon as coding for a given amino acid.
 Although the new amino acid may be different from the
original one, in some cases, transcription results in a
functional protein
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Types of Viral Mutation
A. Point Mutations
2.
Plaque-size mutants:
◦ Mutations here induce changes in the diameter of
the zone of viral lysis seen in infected cell
monolayers.
◦ The mutation usually affects the capsid protein,
allowing easier adsorption to and penetration of
permissive cells.
◦ Mutants with larger plaques grow faster in cell
culture and are usually more virulent in vivo.
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Types of Viral Mutation
A. Point Mutations

Plaques:
◦ Many viruses can be isolated as a result of their
ability to form discrete visible zones, plaques
(areas where cells are killed or altered by the
virus infection) in the host cells
Plaques formed by a phage in a
bacterial culture
Types of Viral Mutation
A. Point Mutations
Wild type
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Mutant 1
Mutant 2
Mutant 3
Types of Viral Mutation
A. Point Mutations
3.
Host-range mutants:
◦ These infect cell types or species not infected by the
wild type strain.
◦ The basic mutation affects either:
 proteins that mediate adsorption to host cells
 or genes that modulate the interplay between cellular
replication and viral expression.
4.
Drug-resistant mutants:
◦ They are not susceptible to antiviral agents
successfully used to treat infections caused by the
wild type strain.
◦ The mutation usually affects an enzyme that is
inhibited by the antiviral drug
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Types of Viral Mutation
A. Point Mutations
5.
Enzyme-deficient mutants:
◦ Strains that lack an enzyme or have a mutant
enzyme.
◦ The mutation may or may not be lethal, depending
on whether it affects an enzyme essential for
multiplication.
6.
Hot mutants:
◦ These grow well at 41oC and are extremely
virulent.
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B. Deletion Mutations
Deletion mutants occur when a whole
segment of the viral genome is lost.
 Defective virus particles are examples of
deletion mutants.

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INTERACTIONS
BETWEEN VIRUSES
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Interactions Between Viral
Genomes

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
This type of interaction occurs in multiply
infected cells i.e., when many virions gain access
to the same cell more or less at the same time.
The co-infecting viruses are usually identical or
closely related.
Because of this multiplicity of infection, the
genomes of the infecting particles may interact
in a variety of ways:
1. Recombination
2. Reassortment
3. Complementation
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1. Recombination

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Recombination is the physical interaction of
virus genomes during superinfectrion (i.e.,
multiple infection) resulting in gene
combinations not present in the original
infecting viruses.
1. Recombination

DNA viruses:
◦ Recombination classically involves doublestranded DNA viruses.
◦ Recombination is insignificant when the
recombination involves two normal genomes, but
recombination may result in the creation of a
complete wild-type genome from two
mutant genomes, providing that the defect is in
different genes (Figure 1)
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Figure 1. Schematic diagram
of recombination between
two double-stranded DNA
molecules. A,B,C and D
designate different genes
(only recombinants are
shown).
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1. Recombination

RNA viruses:
◦ When copying (+) strands into (-) strands to be
used as templates for replication, the RNA
polymerase may “jump” from one (+) strand to
another, producing a hybrid (-) RNA template
(Figure 2).
◦ A similar mechanism may contribute to the
genetic diversity of HIV.
◦ The genome of HIV is made up of two strands of
(+) RNA.
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1. Recombination
In the process of transcribing DNA from RNA,
the reverse transcriptase often “jumps” from
one RNA strand to the other.
 Such jumps are inconsequent if the strands are
identical; however, if two different mutant strains
of HIV manage to infect the same cell, the
progeny virions will contain non-identical
RNA strands.
 Under these conditions, the polymerase “jumps”
may cause the emergence of recombinant
genomes.

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RNA polymerase
Figure 2. Production of
hybrid (recombinant)
RNA molecule by RNA
polymerase jump (A, B, C
and D represent different
genes).
RNA Replicase
Progeny genome
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2. Reassortment
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This mechanism applies only to viruses with
segmented genomes such as, influenza virus.
For example, when a host cell is co-infected with
two different strains of influenza virus, that have
mutated in different segments of their genomes,
a wild-type virus may be formed by
reconstituting a normal set of genome segments,
using segments from both infecting mutants
(Figure 3).
Reassortment is believed to be the major cause
of antigenic shift in influenza virus
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Figure 3. Reassortment between two influenza viruses.
3. Complementation:

This is a process that allows defective viruses
to replicate and spread.
a) If a cell is co-infected by two viruses for
example, one that lacks the gene that codes for
an essential polymerase and another that lacks
the gene that codes for the glycoprotein spikes.
◦ The defective viruses may assist each other in
producing infective progeny by coding for the
structures necessary to assemble non-defective
particles between the two of them.
◦ However the packaged genomes remain
defective.
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3. Complementation:
b)Alternatively, if a defective virus (for example,
one lacking one or several genes essential for its
full replication) co-infects a cell with a
nondefective virus of very similar structure, the
defective genome may be packaged into a viral
particle whose structural components are coded
by the non-defective virus.
◦ This is the way in which defective tumor-causing
retroviruses propagate and spread in nature
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3. Complementation:
c)
Phenotypic mixing and pseudotype formation
are special examples of complementation that
may take place when two closely related
viruses (e.g., poliovirus I and II) coinfect a cell.
◦ Phenotypic mixing:
 In the process of assembly of progeny virions, hybrid capsids
constituted by subunits coded by the two different viruses
may be generated.
◦ Pseudotype formation:
 At the time of encapsulating progeny nucleic acid, viral
genomes of one virus may be encapsulated in the capsids of
the second virus, or vice versa.
 The genetic process resulting in pseudotype formation is
known as phenotypic masking.
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IT INVOLVES NO
ALTERATION IN GENETIC
MATERIAL, the progeny of such
virions will be determined by
which parental genome is
packaged and not by the nature
of the envelope
Pseudotype formation
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3. Complementation:
 Phenotypic masking is reversed when a single
pseudotype infects a host cell.
 That is, if the particle carries a type II genome inside a
type I capsid, the progeny will necessarily carry type II
capsids and genomes, because all progeny components
will be coded by a type II genome (Figure 4).
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Figure 4. Pseudotype
formation. Notice that
pseudotypes return
to their normal states
after single infection of
a host cell. Red and
blue colors represent
different genomes and
different capsid
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Defective interfering (DI)
particles
Viral particles that contain defective genomes
can not multiply on their own.
 Their successful replication depends on
simultaneous infection with infectious
“helper” virus.
 However, the defective particles often
interfere with the replication of the “helper”
virus, hence the designation “defective
interfering particles”.
 High multiplicity of infection at the single-cell
level favors the formation of DI particles.
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Unusual Infectious Agents
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Genomes of nondefective viruses range in size from
2,400,000 bp of double-stranded DNA (Pandoravirus
salinus) to 1,759 bp of single-stranded DNA (porcine
circovirus).
Are even smaller viral genomes possible? Viroids,
satellites, and prions provide answers to these questions.
The adjective “subviral” was coined, in part, because these
agents did not fit into the standard taxonomy schemes for
viruses.
The Subviral RNA Database lists 2,923 nucleotide
sequences for viroids and satellites.
No prion sequences will be found in this database, because
these infectious agents are, remarkably, devoid of nucleic
acid.
Viroids
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A small circular RNA molecules, rod-like secondary
structure, there is no capsid or envelope.
They are associated with certain plant diseases and
transmitted by seeds or pollens.
Viroid RNA does not code for any known proteins.
Some viroids are ribozymes, having RNA enzyme
properties which allow self-cleavage and ligation of
unit-size genomes from larger replication
intermediates.
It has been proposed that viroids are “escaped
introns”
Satellite (virusoids)
They are viroid-like molecules but somewhat
larger.
 They depend on other virus replication for
multiplication” hence-satellite”.
 They are packaged into virus capsid as
passenger.
 When a satellite encodes the coat protein in
which its nucleic acid is encapsidated, it is
referred to as a satellite virus.
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Quasispecies
They are closely-related”but nonidentical”
mutant and recombinant self-replicating RNA
viral genomes.
 Quasispecies are subjected to continuous
genetic variation, competition and selection.
 The structure and dynamics of replicating
RNA enable virus populations to persist in
their host and cause disease.
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