Which proteins are responsible for initiating the switch between the prophage and lytic pathways?

The lambda repressor protein and Cro protein play crucial roles in the decision-making process of bacteriophage lambda when determining whether to enter the lysogenic (prophage) or lytic cycle. The lambda repressor, which is produced when the virus is in the lysogenic cycle, functions to maintain the prophage state by inhibiting the expression of lytic genes. This repression allows the viral DNA to integrate into the bacterial host genome and lie dormant.

When environmental conditions change or the host cell experiences stress, such as DNA damage, the balance between the abundance of the lambda repressor and Cro protein shifts. Cro protein promotes the expression of lytic genes and thus encourages the switch to the lytic cycle, leading to the production of new viral particles and cell lysis. The interplay between these two proteins determines the fate of the viral infection and showcases a classic example of genetic regulation in phage biology.