lysogenesis

Lysogenesis, in bacteriophage biology, is the process by which a temperate bacteriophage establishes a lysogenic state in a bacterial host. In this state, the phage genome is integrated into the host genome (as a prophage) or maintained as an extrachromosomal element and is replicated with the host DNA without causing immediate lysis. Lysogenesis is an alternative to the lytic cycle, and lysogenic phages are termed temperate.

Mechanism: Upon infection, temperate phages may choose lysogeny. Integration is mediated by phage-encoded integrase and excisionase enzymes, with recombination at attachment sites attP on the phage and attB on the bacterial chromosome, yielding prophage with attachments attL and attR. Prophage genes are kept repressed by phage-encoded repressors (for example, the cI repressor), allowing host cell survival and replication of the prophage along with the genome.

Maintenance and induction: The prophage is maintained during cell division. Under stress or DNA damage, the SOS response activates RecA, leading to cleavage of the repressor and excision of the prophage, which can then enter the lytic cycle, producing new phage particles and lysing the host.

Consequences: Lysogenesis can alter host phenotype through lysogenic conversion, where the prophage encodes additional traits such as toxin production or immune evasion. Prophages also facilitate horizontal gene transfer via transduction and influence bacterial evolution. Some phages are temperate; others are strictly lytic and do not participate in lysogenesis.

Examples: The lambda phage of Escherichia coli is a classic model system for lysogenesis. Other temperate phages contribute to virulence in various bacterial pathogens through lysogenic conversion. See also lysogeny, prophage, transduction.