Prophage

A prophage is a phage genome inserted as part of the linear structure of the DNA chromosome of a bacterium. A temperate phage integrated into the host chromosome or existing as an extrachromosomal plasmid. This is a latent form of a bacteriophage in which the viral genes are incorporated into the bacterial chromosomes without causing disruption of the bacterial cell.

Upon detection of host cell damage, the prophage is excised from the bacterial chromosome in a process called prophage induction. After induction, viral replication begins via the lytic cycle. Prophages are important agents of horizontal gene transfer, and are considered to be part of the mobilome.

Prophages and Their Contribution to Host Cell Phenotype
In many bacterial species, prophages figure prominently in the biology of these cells, often conferring key phenotypes that can convert a non-pathogenic strain into a pathogen. The source of these phenotypic changes can be through prophage-encoded toxins, bacterial cell surface alterations, or resistance to the human immune system. Further, prophage integration into the host genome can inactivate or alter the expression of host genes. In addition to these direct genetic alterations associated with the addition or inactivation of genes, prophages can also alter the phenotype of bacteria at the population level by facilitating the spread of favorable genes through transduction.

Prophage Induction of Phage &lambda;
The gene regulatory circuitry of phage &lambda; is among the best-understood circuits at the mechanistic level. This circuitry involves several interesting regulatory behaviors. An infected cell undergoes a decision between two alternative pathways, the lytic and lysogenic pathways. If the latter is followed, the lysogenic state is established and maintained. While this state is highly stable, it can switch to the lytic pathway in the process of prophage induction, which occurs when the host SOS response is triggered by DNA damage.