MUTAGENIC AND RECOMBINOGENIC ACTION OF DNA MONOADDUCTS PHOTOINDUCED BY THE BIFUNCTIONAL FUROCOUMARIN 8‐METHOXYPSORALEN IN YEAST (Saccharomyces cerevisiae)

Abstract—For the same furocoumarin 8‐MOP and the same total number of photoadditions, the genetic activity of DNA monoadducts and a mixture of mono‐ and biadducts photoinduced by the bifunctional furocoumarin 8‐methoxypsoralen (8‐MOP) is compared in the yeastSaccharomyces cerevisiae.In the presence of 8‐MOP, 405 nm irradiation induces only monoadducts, whereas 365 nm irradiation induces mono‐ and biadducts (interstrand cross‐links) in DNA. This is shown by heat denaturation‐renaturation experiments on calf thymus DNA treatedin vitroand by alkaline step elution analysis of DNA from treated yeast cells. For the same photobinding of tritiated 8‐MOP to DNA in diploid yeast, about 20 times higher doses are needed with 405 nm than with 365 nm irradiation. Re‐irradiation experiments reveal that part of the monoadducts induced by 8‐MOP and 405 nm irradiation can be effectively converted into DNA interstrand cross‐links by exposures to 365 nm radiation after washing‐out of unbound 8‐MOP molecules. 8‐MOP and 405 nm irradiation induce per lethal hit cytoplasmic “petite” mutations in yeast as efficiently as the monofunctional furocoumarin 3‐carbethoxypsoralen (3‐CPs) and 365 nm irradiation, both treatments being much more efficient than 8‐MOP and 365 nm irradiation. At equal survival, treatments with 8‐MOP and 405 nm radiation are clearly less efficient than treatments with 8‐MOP and 365 nm radiation for the induction of forward (CAN*) and reverse (HIS+) mutations in haploïd yeast and for the induction of mutations (ILV+) and genetically aberrant colonies including mitotic crossing‐over in diploid yeast. The two treatments are equally efficient for the induction of mitotic gene conversion. At equal photobinding of 8‐MOP, the monoadducts induced by 405 nm irradiation are found less effective than the mixture of mono‐and biadducts induced by 365 nm irradiation for the induction of cell killing, mutations and mitotic recombination.