Viral Replication: Lysogenic Cycle

Upon infection, the linear phage genome circularizes to prevent degradation by host nucleases. The circular DNA then integrates into the bacterial chromosome via site-specific recombination at a specific attachment site, forming a dormant prophage. This integration allows the viral DNA to persist within the host without triggering immediate cell lysis.

The cI repressor protein binds to phage operator regions, blocking transcription of genes required for virion assembly and host lysis. This repression maintains prophage dormancy while allowing normal bacterial replication. Each time the bacterium divides, the prophage is duplicated along with the host DNA, ensuring stable vertical transmission across generations.

Lysogenic bacteria acquire immunity against superinfection by the same phage, as the repressor prevents additional infections. Prophages also enable lysogenic conversion, introducing new bacterial traits such as toxin production. Additionally, prophages facilitate horizontal gene transfer, promoting genetic diversity among bacterial populations and enhancing overall fitness.

Environmental stress conditions such as UV radiation exposure, starvation, or chemical damage can trigger prophage excision from the bacterial chromosome. Once excised, the prophage enters the lytic cycle, where viral genes are expressed, new virions are assembled, and the host cell ultimately undergoes lysis, releasing progeny phages.

Phage-encoded integrase catalyzes site-specific recombination between the phage attachment site and the bacterial attachment site (attB in E. coli). This specialized enzyme facilitates precise integration of the viral genome into the host chromosome at predetermined locations, ensuring stable prophage formation and predictable inheritance patterns.

Yes, some prophages can spontaneously excise at low frequency even without external stressors. This spontaneous excision represents a dynamic aspect of lysogeny, where selective pressures and random molecular events influence the long-term persistence of prophages within bacterial populations and their evolutionary impact.

Bacteriophage lambda infects Escherichia coli and demonstrates the complete lysogenic cycle, including genome circularization, site-specific integration, repressor-mediated gene silencing, and prophage stability. Its well-characterized molecular mechanisms and regulatory proteins make it an ideal system for understanding how temperate phages balance viral persistence with host cell viability.