8‐METHOXYPSORALEN‐PHOTOINDUCED DNA CROSSLINKS AS DETERMINED IN YEAST BY ALKALINE STEP ELUTION UNDER DIFFERENT REIRRADIATION CONDITIONS. RELATION WITH GENETIC EFFECTS

— DNA damage induced by 8‐methoxypsoralen (8‐MOP) plus near UV light (UVA) was analyzed in diploid yeast using the alkaline step elution technique. The presence of 8‐MOP and UVA induced DNA interstrand cross‐links was revealed by the increase of DNA retained on elution filters as compared to untreated controls. The fraction of DNA retained on filters increased linearly with UVA dose. The amount of cross‐links was estimated from the fraction of DNA retained on filters using a dose of m̀‐radiation leading to a number of DNA strand breaks at least equivalent to the number of 8‐MOP induced photoadducts. When 8‐MOP treated cells were exposed to monochromatic light, 365 nm light induced monoadducts and cross‐links whereas 405 nm light induced only monoadducts. When submitting 8‐MOP plus 405 nm light treated cells to 365 nm irradiation, after removal of unbound 8‐MOP by washing, a portion of 8‐MOP plus 405 nm light induced monoadducts was converted into cross‐links. The amount of monoadducts transformed into cross‐links was dependent on the dose of 365 nm irradiation up to a maximum likely to correspond to the number of suitably positioned furan‐side monoadducts that could be converted into biadducts. When 8‐MOP plus 365 nm light treated cells were reirradiated with 365 nm light, following the same protocol, the maximum level of cross‐linking obtainable in yeast was lower than that obtained with 8‐MOP in a 405 nm plus 365 nm reirradiation protocol. In the presence of 8‐MOP single exposures to 405 nm light were found to be only slightly genotoxic. However, when followed by second exposures to 365 nm light, a dose‐dependent increase in genetic effects, i.e. mutation and gene conversion, was observed in parallel to the induction of DNA crosslinks. These results stress again the prominent role of DNA cross‐links in the genotoxicity of 8‐MOP.