Inactivation and mutagenesis by phototoxins using Escherichia coli strains differing in sensitivity to near- and far-ultraviolet light

Four Escherichia coli strains carrying all the possible combinations of genes controlling sensitivity to near-UV (NUV;nur versus nur⁺) and far-UV (FUV;uvrA6 versus uvrA⁺) were inactivated with broad-spectrum NUV together with specific phototoxins. The inactivation kinetics of the four strains are consistent with the previous reports that psoralen and angelicin inactivation is based on the formation of DNA adducts, while xanthotoxin (8-MOP) inactivation is based on the combined effects of DNA adduct formation and oxygen-dependent photodynamic action. At sufficiently high NUV fluences, xanthotoxol (8-HOP) induces lethal DNA lesions in an excision-deficient (uvrA6) strain. Inactivation by alpha-terthienyl plus NUV involves strictly membrane damage since the genes controlling the sensitivity to either NUV or FUV have no effect on inactivation kinetics. Using mutation to histidine independence (his-4⁺) in the presence of NUV as a measure of mutagenicity by phototoxins, psoralen and xanthotoxin are mutagenic, angelicin is less mutagenic, and xanthotoxol and alpha-terthienyl are not mutagenic. None of the phototoxins tested in the presence of NUV were as mutagenic as FUV. Imperatorin and berberine were neither phototoxic nor mutagenic in this assay system. This assay thus provides a rapid qualitative screening procedure to identify the mode of action and mutagenicity of plant phototoxins with potential insecticidal properties.