Introduction to Phage Strategies

A virus consists of a nucleic acid genome contained in a protein coat. In order to reproduce, the virus must infect a host cell. The typical pattern of an infection is to subvert the functions of the host cell for the purpose of producing a large number of progeny viruses. Viruses that infect bacteria are generally called bacteriophages, often abbreviated as phages or simply ϕ.
Usually, a phage infection kills the bacterium. The process by which a phage infects a bacterium, reproduces itself, and then kills its host is called lytic infection. 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. For some phages, called virulent phages, this is their only strategy for survival.
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. They also have an alternative form of existence, though, in which the phage genome is present in the bacterial genome in a latent form known as a prophage. This form of propagation is called lysogeny, and the infected bacteria are known as lysogens. Phages that follow this pathway are called temperate phages.
In a lysogenic bacterium, the prophage is inserted, or recombined, 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 other phage particles of the same type.
Immunity is established by a single integrated prophage, so in general a bacterial genome contains only one copy of a prophage of any particular type.
Transitions occur between the lysogenic and lytic modes of existence. FIGURE .1 shows that when a temperate 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 the phage and the bacterium.

FIGURE .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.
A prophage is freed from the restrictions of lysogeny by a process called induction. First, the phage DNA is released from the bacterial chromosome by another recombination event called excision; the free DNA then proceeds through the lytic pathway.

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. The transition between the two lifestyles is accomplished by the establishment of repression (lytic cycle to lysogeny) or by the relief of repression (induction of lysogen to lytic phage). These regulatory processes provide a wonderful example of how a series of relatively simple regulatory actions can be built up into complex developmental pathways.

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