Viral Replication: Lysogenic Cycle

The lysogenic cycle is a crucial viral replication strategy that allows bacteriophages to persist within host cells without immediately destroying them. This process is primarily observed in temperate phages, such as bacteriophage lambda (λ), which infects Escherichia coli. The cycle allows the viral genome to persist across bacterial generations while keeping host cells viable.

Integration of the Viral Genome

Upon infection, bacteriophage lambda attaches to the bacterial surface and injects its genetic material into the cytoplasm. The linear phage genome undergoes circularization to prevent host nucleases from degrading it. A specialized process known as integrase-mediated recombination, facilitated by phage-encoded integrase, enables the viral DNA to integrate into the bacterial chromosome at a specific attachment site (attB in E. coli). The resulting prophage remains dormant within the host genome.

During lysogeny, regulatory proteins suppress lytic gene expression. The cI repressor protein binds to phage operator regions, preventing transcription of genes required for virion assembly and host lysis. This repression ensures the stability of the prophage while allowing bacterial proliferation. Each time the bacterium replicates, the prophage is duplicated along with the host DNA, ensuring vertical transmission of the viral genome. Although lysogens generally remain stable, some prophages can spontaneously excise at a low frequency, even without external stressors.

Lysogeny offers evolutionary benefits to the host bacterium. The presence of a prophage grants immunity against superinfection by the same phage, as the repressor prevents additional infections. Lysogenic conversion can enhance bacterial fitness by introducing new traits, such as toxin production in Vibrio cholerae and Corynebacterium diphtheriae. Furthermore, prophages contribute to horizontal gene transfer, facilitating genetic diversity among bacterial populations.

Lysogeny represents a dynamic interaction between bacteriophages and bacteria, balancing viral persistence with the potential for rapid proliferation. While prophages are generally stably inherited, selective pressures or spontaneous excision events can influence their long-term persistence in bacterial populations.