Rather they are associated with a selfish genetic

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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 of phage chromosome Injection of phage chromosome Integration into bacterial chromosome 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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