12 PHAGE STRATEGIES (Full Edition)
1 Introduction
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- Lytic infection of a bacterium by a phage ends
in the destruction of the bacterium with release of progeny phage.
- Lysis describes the death of bacteria at the end
of a phage infective cycle when they burst open to release the progeny of an infecting
phage (because phage enzymes disrupt the bacterium's cytoplasmic membrane or cell
wall). The same term also applies to eukaryotic cells; for example, when infected
cells are attacked by the immune system.
- Prophage is a phage genome covalently integrated
as a linear part of the bacterial chromosome.
- Lysogeny describes the ability of a phage to survive
in a bacterium as a stable prophage component of the bacterial genome.
- Integration of viral or another DNA sequence describes
its insertion into a host genome as a region covalently linked on either side to
the host sequences.
- Immunity in phages refers to the ability of a
prophage to prevent another phage of the same type from infecting a cell. It results
from the synthesis of phage repressor by the prophage genome.
- Immunity in plasmids describes the ability of
a plasmid to prevent another of the same type from becoming established in a cell.
It results usually from interference with the ability to replicate.
- Induction of prophage describes
its entry into the lytic (infective) cycle as a result of destruction of the lysogenic
repressor, which leads to excision of free phage DNA from the bacterial chromosome.
- The excision of phage or episome or other sequence
describes its release from the host chromosome as an autonomous DNA molecule.
- The excision step in an excision-repair system
consists of removing a single-stranded stretch of DNA by the action of a 5′-3′
exonuclease.
- A plasmid is a circular, extrachromosomal DNA.
It is autonomous and can replicate itself.
- An extrachromosomal genome in a bacterium is a
self-replicating set of genes that is not part of the bacterial chromosome. In many
cases, the genes are necessary for bacterial growth under certain environmental
conditions.
- An episome is a plasmid able to integrate into
bacterial DNA.
- Immunity refers to the ability of certain transposons
to prevent others of the same type from transposing to the same DNA molecule.
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Some phages have only a single strategy for survival. On infecting a susceptible
host, they subvert its functions to the purpose of producing a large number of progeny
phage particles. As the result of this lytic infection, the host bacterium dies.
In the typical lytic cycle, the phage DNA (or RNA) enters the host bacterium, its
genes are transcribed in a set order, the phage genetic material is replicated,
and the protein components of the phage particle are produced. Finally, the host
bacterium is broken open (lysed) to release the assembled progeny particles
by the process of lysis.
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Other phages have a dual existence. They are able to perpetuate themselves via the
same sort of lytic cycle in what amounts to an open strategy for producing as many
copies of the phage as rapidly as possible. But they also have an alternative form
of existence, in which the phage genome is present in the bacterium in a latent
form known as prophage. This form of propagation is called lysogeny.
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In a lysogenic bacterium, the prophage is inserted into the bacterial genome, and
is inherited in the same way as bacterial genes. The process by which it is converted
from an independent phage genome into a prophage that is a linear part of the bacterial
genome is described as integration. By virtue of its possession of a prophage, a
lysogenic bacterium has immunity against infection by further phage particles of
the same type. Immunity is established by a single integrated prophage, so usually
a bacterial genome contains only one copy of a prophage of any particular type.
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Figure 12.1
Lytic development involves the reproduction of phage particles with destruction
of the host bacterium, but lysogenic existence allows the phage genome to be carried
as part of the bacterial genetic information.
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Transitions occur between the lysogenic and lytic modes of existence. Figure 12.1
shows that when a phage produced by a lytic cycle enters a new bacterial host cell,
it either repeats the lytic cycle or enters the lysogenic state. The outcome depends
on the conditions of infection and the genotypes of phage and bacterium.
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A prophage is freed from the restrictions of lysogeny by the process called induction.
First the phage DNA is released from the bacterial chromosome by excision; then
the free DNA proceeds through the lytic pathway.
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The alternative forms in which these phages are propagated are determined by the
regulation of transcription. Lysogeny is maintained by the interaction of a phage
repressor with an operator. The lytic cycle requires a cascade of transcriptional
controls. And the transition between the two life-styles is accomplished by the
establishment of repression (lytic cycle to lysogeny) or by the relief of repression
(induction of lysogen to lytic phage).
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Another type of existence within bacteria is represented by plasmids.
These are autonomous units that exist in the cell as extrachromosomal genomes.
Plasmids are self-replicating circular molecules of DNA that are maintained in the
cell in a stable and characteristic number of copies; that is, the number remains
constant from generation to generation.
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Some plasmids also have alternative life-styles. They can exist either in the autonomous
extrachromosomal state; or they can be inserted into the bacterial chromosome, and
then are carried as part of it like any other sequence. Such units are properly
called episomes (but the terms "plasmid" and "episome" are sometimes
used loosely as though interchangeable).
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Like lysogenic phages, plasmids and episomes maintain a selfish possession of their
bacterium and often make it impossible for another element of the same type to become
established. This effect also is called immunity, although the basis for plasmid
immunity is different from lysogenic immunity. (We discuss the control of plasmid
perpetuation in The replicon.)
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Figure 12.2
Several types of independent genetic units exist in bacteria.
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Figure 12.2 summarizes the types of genetic units that can be propagated in bacteria
as independent genomes. Lytic phages may have genomes of any type of nucleic acid;
they transfer between cells by release of infective particles. Lysogenic phages
have double-stranded DNA genomes, as do plasmids and episomes. Some plasmids and
episomes transfer between cells by a conjugative process (involving direct contact
between donor and recipient cells). A feature of the transfer process in both cases
is that on occasion some bacterial host genes are transferred with the phage or
plasmid DNA, so these events play a role in allowing exchange of genetic information
between bacteria.
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Last Revised on April 28, 2004
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©Jones and Bartlett Publishers (2007)
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