Non-photoreactivating repair of mutational lesions induced by ultraviolet and ionizing radiations in Escherichia coli

Abstract

The action of one established and two presumed repair systems on premutational lesions induced by ionizing and ultraviolet radiations has been studied int he tryptophan auxotrops Escherichia coli B/r WP2 and E. coli WP2 hcr t− (which has a reduced ability to excise thymine dimers). Premutational lesions induced by ionizing radiation are not susceptible to excision-repair nor to the mutation-frequency decline which occurs with UV lesions in minimal media, but are removed by a slow-acting process at 16°. Premutational lesions induced by UV are removed by excision-repair, by mutation-frequency decline both at 37° in minimal medium and at 16° even on plates enriched with nutrient broth, and by a similar low-temperature process to that which acts at 16° on lesions induced by ionizing radiation. There is some evidence that mutation-frequency decline may be related to excision-repair, for example by the involvement of a common enzyme. Alternatively mutation-frequency decline could merely reflect the excision-repair of premutational lesions which are more amenable to excision than thymine dimers. The slow acting process at 16° appears to be quite distinct from excision-repair and mutation-frequency decline since, unlike these, it occurs normally in WP2 hcr −.