INHIBITION OF DNA REPAIR SYNTHESIS BY SUNLIGHT

— DNA repair synthesis as determined by thymidine incorporation in the presence of hydroxyurea reached a much lower maximum level after solar compared with UVC exposure in five human melanoma cell lines, in HeLa cells, and in two human fibroblast strains. This finding was confirmed by determination of unscheduled DNA synthesis where both the number of labelled nuclei and grain count per nucleus were lower in sun‐exposed cells. In a cloned human melanoma line (MM253cl), glass‐filtered sunlight inhibited UVC repair synthesis, and solar UVB alone induced a higher level of repair synthesis than either complete sun or solar UVA plus solar UVB. The fluence response of filtered sunlight for inhibition of UVB (sunlamp) and UVC showed that most inhibition was obtained at low fluences (5‐10 min), further exposure giving a plateau at 40% of the original level. Ultraviolet C and sunlight inactivated adenovirus 5 giving F 0 values for virus survival 40‐fold higher than for cell survival. Replication of either UVC‐ or solar‐irradiated virus was not affected by prior irradiation of cells with glass‐filtered sunlight. Stathmokinetic analysis of cell cycle progression by DNA flow cytometry showed that UVC and sunlamp UVB retarded cell movement from the G1 and S phases whereas equitoxic sunlight and glass‐filtered sunlight (nontoxic) had no effect. These results indicate that solar UVA at low environmental fluences partially inhibits UVB repair synthesis in a range of human cell types but does not affect the replication of a UVB‐ or UVC‐damaged virus when applied to the genome alone or to the host cell.