Lysogeny is the harbouring of a quiescent bacteriophage genome by a growing bacterial host. Coliphage lambda (the paradigmatic example) is treated in most detail and other phages are compared to it. The role of regulatory proteins in determining the fate of the infected cell is indicated as well as variations seen in other phages. Insertion of lambda deoxyribonucleic acid (DNA) into the host chromosome by circularisation and site-specific recombination is described and compared to transposon-like insertion by phage Mu, plasmid formation by phage P1 and single-strand insertion by the filamentous cholera phage CTX. The mechanism of shifting from lysogenic to lytic growth due to repressor degradation mediated by RecA protein activated by single-stranded DNA (ssDNA) is outlined. Hypotheses on the role of lysogeny in nature are discussed.

Key Concepts:

• Latent viruses, whose genomes persist in the host over long periods in absence of ongoing infection, are common throughout the living world and are implicated in human maladies such as shingles.
  
• Site-specific recombination, which entails breakage and joining of DNA at specific sequences, is used by viruses and plasmids and facilitates separation of daughter chromosomes in bacterial cell division.
  
• DNA looping occurs when protein molecules which bind to noncontiguous DNA sites also bind to each other, bringing together DNA sites that are normally some distance apart.
  
• In transposition, some DNA elements, including viral genomes, move from one site to another by action of transposases encoded by the element.
  
• Some latent viruses can perpetuate their genomes without integration into the chromosome by plasmid formation, where viral genomes replicate autonomously to keep pace with cell division.
  
• Prophage induction is the activation of a prophage to produce active viruses by treating lysogenic cells with various agents.