EFFECTS OF 8‐METHOXYPSORALEN AND NEAR ULTRAVIOLET RADIATION ON THE SURVIVAL OF THE LOWER EUKARYOTE D. Discoideum

Seven axenic wild‐type and repair‐deficient mutant strains of the cellular slime mold Dictyostelium discoideum have been treated with the furocoumarin 8‐methoxypsoralen (8‐MOP) up to 50 μg/mζ and then exposed to near ultraviolet light (UVA 320‐400 nm) up to 21 kJ/m2. Fluence‐response survival curves exhibit shoulders at lower fluences and an exponential lethal response at higher fluences. Neither the psoralen alone nor the irradiation alone produced any measurable lethal effect. Wild‐type strains, which show resistance to 254 nm UV and gamma radiation, also show resistance to psoralen plus UVA. The moderate sensitivity of a rad D repair‐deficient mutant strain and the extreme sensitivity of a rad B mutant strain to 8‐MOP plus UVA parallel their responses to UV and gamma radiation. However a rad C mutant which is sensitive to UV, exhibits wild‐type response to photoactivated psoralen.