Ultraviolet reactivation of the single stranded DNA phage phiX 174

When UV-irradiated phiX174 was grown in pre-irradiated host cells of various strains, "ultraviolet reactivation" (UVR) was observed only in recombination proficient strains such as E. coli C (uvrA+ recA+) and HF4704 (uvrA- recA+), but not in the recombination deficient strain HF4712 (uvrA+ recA-). By increasing the multiplicity of infection, no rise in the amount of such reactivation was observed. From the study of the neutral and alkaline sucrose gradient sedimentation patterns of DNA samples extracted from unirradiated cells infected with unirradiated phage, it appears that after the conversion of the viral single stranded (SS) DNA to the double stranded form (DS), "nicks" or scissions were produced on it within all three strains, which were ultimately sealed up in the recA+ but persisted within the recA- host cells. When UV-irradiated phage infected unirradiated host cells, such nicking of the DS DNA appeared to be much more extensive in uvrA+ recA+, but slightly reduced in uvrA+ recA- and severely suppressed in uvrA- recA+ strains. When the host cells were also UV-irradiated, the conversion of the infecting viral SS DNA to DS DNA as well as its subsequent nicking were reduced in all the three strains to a much greater extent. Although nicking of the DS DNA molecule is an essential step even in the normal intracellular replication of phiX DNA, the production and the sealing up of such nicks appear not to have any positive correlation with UVR of these phages. A drastic reduction in nicking due to pre-irradiation of the host cells might, however, mean slowing down of the replication of the damaged parental RF molecules which would facilitate their repair perhaps through recombination with the homologous parts of the host genome.