QUANTITATIVE EVIDENCE FOR ENZYMATICALLY-INDUCED DNA DOUBLE-STRAND BREAKS AS LETHAL LESIONS IN UV IRRADIATED pol+ AND polAl STRAINS OF E. COLI K-12

Abstract— We have recently reported that DNA double-strand breaks arise enzymatically during the course of excision repair in uvr ⁺ strains of Escherichia coli K-12. Survival curves for ultraviolet (UV) irradiated E. coli K-12 pol⁺ (JG139) and polA1 (JG138) strains have a pronounced shoulder region. The regions of the survival curves at which killing approaches exponential correspond to the fiuences at which DNA double-strand breaks (assumed to be lethal events) accumulate linearly. Reducing the number of UV photoproducts either by photoreactivation or fluence fractionation results in an increase in survival and a decrease in the yield of DNA double-strand breaks in both strains. These data support the hypothesis that enzymatically-induced DNA double-strand breaks may be the lesion ultimately responsible for UV-induced cell killing in the pol⁺ strain of E. coli K-12. and perhaps also in the polA1 strain.