7.3.7B: The Lysogenic Life Cycle of Bacteriophages
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- Ying Liu
- City College of San Francisco
Describe the lysogenic life cycle of temperate phages (including spontaneous induction).
Define the following:
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temperate phage
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lysogen
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prophage
- temperate phage
- lysogen
- prophage
Bacteriophages capable of a lysogenic life cycle are termed temperate bacteriophages. When a temperate bacteriophage infects a bacterium, it can either replicate by means of the lytic life cycle and cause lysis of the host bacterium, or, it can incorporate its DNA into the bacterium's DNA and become a noninfectious prophage (see Figure \(\PageIndex{1}\)) . In the latter case, the cycle begins by the bacteriophage adsorbing to the host bacterium or lysogen and injecting its genome as in the lytic life cycle (see Figure \(\PageIndex{2}\) and Figure \(\PageIndex{3}\) ). However, the bacteriophage does not shut down the host cell. Instead, the bacteriophage DNA inserts or integrates into the host bacterium's DNA (see Figure \(\PageIndex{4}\)) . At this stage the virus is called a prophage. Expression of the bacteriophage genes controlling bacteriophage replication is blocked by a repressor protein, and the phage DNA replicates as a part of the bacterium's DNA so that every daughter bacterium now contains the prophage (see Figure \(\PageIndex{5}\)) .
The number of viruses infecting the bacterium as well as the physiological state of the bacterium appear to determine whether the temperate bacteriophage enters the lytic cycle or becomes a prophage.
In about one out of every million to one out of every billion bacteria containing a prophage, spontaneous induction occurs. The bacteriophage genes are activated and new bacteriophages are produced by the lytic life cycle (see Figure \(\PageIndex{5}\)A , Figure \(\PageIndex{6}\) , Figure \(\PageIndex{7}\) , Figure \(\PageIndex{8}\) , and Figure \(\PageIndex{9}\) ).