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Unformatted text preview: etic element harboured by the bacterium,
i.e., the bacterium can only cause disease if is infected with a virus or plasmid (Table 9-2). FOOD-286 Circularization
chromosome Injection of
phage chromosome Integration into
by recombination No integration Cell division Many lysogenic daughter cells Cell division
Many lysogenic daughter cells Figure 9-2: Modes of phage dormancy in lysogenic infection Importance of
prophage Sometimes when a temperate phage (Chapter 5) infects a bacterium, a lytic infection does
not result. Instead, the phage enters a dormant state inside the host cell called lysogeny.
The dormant phage is called a prophage. There is a complex regulatory mechanism that
determines whether a temperate phage enters a lytic or lysogenic cycle. Although the
information needed for the virus to assemble new virions is not expressed, some of the
prophage genes are expressed, for example, those needed to maintain the virus in the
prophage state. This usually involves the expression of a phage gene for a specific
regulatory protein that “turns-off” the expression of genes required for a lytic infection.
Sometimes when a bacterium harbours a prophage its phenotype changes due to the
expression of some prophage genes. When the expression of prophage genes changes the
phenotype of the bacterium, the phenomenon is called phage conversion. Lysogeny and
phage conversion are very important to human health because phage sometimes carry genes
that make a bacterium harmful to humans. We have already seen one one example of this:
the strain of C. botulinum that causes botulism only does so because it harbours a
prophage. It is the phage genome that carries the gene for the botulism toxin. Similarly,
Vibrio cholerae only produces the protein toxin that causes cholera because it harbors a
prophage. Cholera continues to kill thousands of persons every year.
Depending on the particular phage, the phage DNA remains in circular form in the
cytoplasm or the phage DNA becomes joined (integrated) to the bacterial DNA by
homologous recombination (Figures 9-2 and 9-3).
Phage chromosomes and their host cell chromosomes often share share short specific
nucleotide sequences that provide regions of homology for recombination. Homologous
recombination between two ds circular DNAs leads the joining of the two molecules
(Figure 9-3). The mechanism by which this occurs, although similar, is more complicated
than that involving ss donor DNA (Chapter 5). The details of this mechanism are not
considered in MICB201 except to say it also involves complementary base
pairing between single strands of donor and recipient DNA, and requires the action of the
RecA protein. FOOD-287 Short regions
of homology Bacterial
chromosome Prophage Phage
chromosome Figure 9-3: Integration of phage into a bacterial chromosome Regardless whether the prophage is integrated into the chromosome or exists as an
independent entity, it is replicated by host enzymes and passed on to daughter cells
(Figure 9-2). Most temperate phage do not stay as prophage indefinitely...
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- Fall '12