Differential Causes of Mutation and Killing in Escherichia Coli After Psoralen Plus Light Treatment: Monoadducts and Cross‐links

On treatment with 8‐methoxypsoralen plus near UV light, an excisionless (uvrB‐) strain of Escherichia coli showed about 3– and 10 times higher sensitivities to killing and mutation, respectively, than its parental strain. On re‐irradiation with near UV in the absence of unbound psoralen, the uvrB‐ strain pretreated with psoralen plus near UV showed a decrease in both survival and mutation. After treatment with psoralen plus near UV, re‐irradiation of T7 DNA in the absence of unbound psoralen caused an increase in the cross‐linked fraction with an equivalent decrease in the non‐cross‐linked fraction. From these and previous results, we conclude that monoadducts produced by treatment with psoralen plus near UV are converted to cross‐links by further irradiation and that, in E. coli, monoadducts are responsible for the mutation induced by psoralen‐plus‐light whereas cross‐links are the major cause of its lethal action.