PSORALENS CLEAVE pBR322 DNA UNDER ULTRAVIOLET RADIATION

Abstract— supercoiled (SC) pBR322 was used to probe the recent claim that 5‐geranoxylpsoralen (5‐GOP) did not photoreact with DNA. Contrary to expectations, 5‐GOP was found to damage DNA in the presence of UV‐A through two competing pathways: (a) single strand breaks, identified by the conversion of supercoiled into open circular and linear DNA, and (b) cross‐linking, revealed by the fluence‐dependent decrease in the extent of denaturation of the double stranded supercoiled DNA to single stranded circular DNA. In addition, a fluence‐dependent modification reduced the ability of the restriction enzyme EcoR I to linearize the photosensitized DNA, and alkali‐labile lesions were generated. Psoralen, 5‐methoxypsoralerl, and 8‐methoxypsoralen, which are well‐known to undergo cycloaddi‐tion to DNA, had a more pronounced effect on supercoiled DNA. Single strand breaks occurred more readily than with 5‐GOP, and the surviving SC form remaining had reduced electrophoretic mobility in agarose gels. In all cases, the DNA damage was more prominent when oxygen was absent.