Lysogeneesi

Lysogeneesi, also known as lysogeny, is a phase of the bacteriophage life cycle in which a temperate phage integrates its genome into the host bacterium's chromosome, becoming a prophage. During lysogeneesi, the phage genome is replicated with the host DNA and no new virions are produced until induction occurs.

Integration occurs via phage-encoded integrase at attachment sites attP and attB, resulting in a stable prophage state maintained by repressor proteins (such as CI) that silence lytic genes. The host gains immunity to closely related phages (superinfection immunity) and the prophage can be inherited across bacterial cell divisions.

Under stress, the SOS response can trigger induction: RecA-mediated cleavage of the CI repressor lifts repression, allowing prophage excision and entry into the lytic cycle, followed by replication, assembly of phage particles, and cell lysis.

Lysogeneesi contributes to bacterial evolution by lysogenic conversion, in which prophage genes alter host phenotype, including virulence factors like toxins in some pathogens, and can mediate horizontal gene transfer. Prophages also serve as reservoirs of genetic diversity and can influence host fitness in various environments.

Classic examples include the lambda phage in Escherichia coli and a variety of other temperate phages across bacterial species. The lysogenic state represents an alternative strategy to the lytic cycle, enabling persistence of the phage genome within hosts and shaping microbial ecosystems.