The effect of ultraviolet light on the establishment of lysogeny.

Abstract

Abstract The resistance to ultraviolet light (UV) of cells of Shigella dysenteriae, infected with the temperate phage P2 or with various temperate and virulent mutants of P2, was measured at various times in the course of the latent period. The resistance of cells infected with the virulent phages or of the lysing fraction of cells infected with the temperate phages (lytic complexes) was measured as the survival of the ability to form plaques, while the resistance of the lysogenizing fraction of cells infected with the temperate phages was measured as the survival of the ability to give origin to lysogenic colonies. All lytic complexes go through a stage during which their resistance to UV increases rapidly, although those infected with the virulent phages do so much earlier in the latent period than those infected with the temperate phages. The apparent UV resistance of lysogenizing complexes increases soon after infection, but decreases again toward the end of the latent period. If cells, infected with a temperate mutant that has a greatly reduced ability to lysogenize as compared to the wild type P2, are irradiated early in the latent period, shortly before the resistance of the lytic complexes begins to change, the absolute number of lysogenizing complexes increases rather than decreases as a result of the irradiation. These facts suggest that the frequency of lysogenization with phage P2 can be increased by irradiation of the infected cells with ultraviolet light at a critical time in the early part of the latent period. For 1 to 2 hours after the end of the latent period, cells that survive infection and give rise to lysogenic progeny have a lower UV resistance than established lysogenic strains. Possible interpretations of this observation are discussed.