Toxicity and repair of psoralen adducts in bacteriophage T7

Psoralens react photochemically with DNA to form interstrand crosslinks as well as two types of monoadduct (furan-side and pyrone-side adducts). To investigate the relative roles of these adducts in toxicity, we have studied the interaction of 4,5',8-trimethylpsoralen (TMP) and 8-methoxypsoralen (8-MOP) with bacteriophage T7. These two derivatives differ in the fraction of pyrone-side monoadducts formed, TMP producing very small amounts of this type of adduct. The results show similar phage survival for the two psoralen analogs at equivalent numbers of crosslinks per DNA molecule. However, the survival fraction of treated phage is significantly lower than the fraction of noncrosslinked DNA molecules. Phage survival decreases after secondary irradiation which is used to transform monoadducts into crosslinks, but this decrease is not due solely to crosslinks; at doses beyond that required to transform all crosslinkable monoadducts into crosslinks, phage survival continues to decrease, pointing to the production of other genotoxic lesions during secondary irradiation. These results indicate that, although crosslinks can kill phage T7, as shown by the secondary irradiation results, they are not sufficient in number to explain the psoralen toxicity after primary irradiation. Therefore monoadducts, both furan-side and pyrone-side types, must in large part be responsible for phage inactivation.