1. Introduction

11.1 Introduction

Key terms defined in this section
Episome is a plasmid able to integrate into bacterial DNA. Epistasis
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.
Induction refers to the ability of bacteria (or yeast) to synthesize certain enzymes only when their substrates are present; applied to gene expression, refers to switching on transcription as a result of interaction of the inducer with the regulator protein.
Lysogeny describes the ability of a phage to survive in a bacterium as a stable prophage component of the bacterial genome.
Lytic infection of bacteria by a phage ends in destruction of bacteria and release of progeny phage.
Plasmid is an autonomous self-replicating extrachromosomal circular DNA.
Prophage is a phage genome covalently integrated as a linear part of the bacterial chromosome.

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.


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.


In a lysogenic bacterium, the prophage is integrated into the bacterial genome, and is inherited in the same way as bacterial genes. 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.




Figure 11.1 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.

Transitions occur between the lysogenic and lytic modes of existence. Figure 11.1 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.


A prophage is freed from the restrictions of lysogeny by the process called induction, in which it is excised from the bacterial genome, to generate a free phage DNA that 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. And 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).


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.


Some plasmids also have alternative lifestyles. 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).


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 12 The replicon.)




Figure 11.2 Several types of independent genetic units exist in bacteria.

Figure 11.2 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.




Genes VII
Genes VII
ISBN: B000R0CSVM
EAN: N/A
Year: 2005
Pages: 382

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