Mutagenesis in Escherichia coli IV. Photosensitization to near ultraviolet light by 8-methoxypsoralen

Abstract

Suppressor mutations and true reversions of Escherichia coli WP2 were produced in roughly equal numbers by near ultraviolet light (NUV) (≈ 360 nm) in the presence of 8-methoxypsoralen (8-MOP). Both types of mutation are probably the result of base-pair transitions. Neither NUV nor 8-MOP was mutagenic by itself. Both premutational and potentially lethal damage produced by 8-MOP plus NUV was in the DNA and largely excisable as judged by the comparative sensitivities of strains possessing and lacking excision-repair capacity. It was not photoreactivable. A “reckless” Rec- strain was sensitive to the lethal effect of 8-MOP plus NUV and was not detectably mutable; mutability was also lacking in an Exr- mutant. The mechanism of mutagenesis is thus similar to that for far ultraviolet light alone although there are some differences, indicated by the much smaller proportion of suppressor mutants and by the very small “broth effect”, which are largely attributable to an independent physiological effect of 8-MOP and, to a lesser extent, NUV. 8-MOP may represent a new type of antimutagen for suppressor mutations arising from potentially excisable damage.