Repair of near-ultraviolet (365 nm)-induced strand breaks in Escherichia coli DNA. The role of the polA and recA gene products

Abstract

The action of near-ultraviolet (UV-365 nm) radiation in cellular inactivation (biological measurements) and induction and repair of DNA strand breaks (physical measurements) were studied in a repair-proficient strain and in polA-, recA-, uvrA-, and polA uvrA-deficient strains of Escherichia coli K-12. The induction of breaks in the polA and polA uvrA strains was linear with dose (4.0 and 3.7 X 10(-5) breaks/2.5 X 10(9) daltons/Jm-2, respectively). However, in the recA-, uvrA-, and repair-proficient strains, there was an initial lag in break induction at low doses and then a linear induction of breaks at higher doses with rates of 4.6, 2.8, and 3.2 X 10(-5) breaks/2.5 X 10(9) daltons/Jm-2, respectively. We interpret these strain differences as indicating simultaneous induction and repair of breaks in polymerase 1 (polA)-proficient strains under the 0 degrees C, M9 buffer irradiation conditions that, for maximum efficiency, require both the polA and recA gene products. Strand-break rejoining also occurred at 30 degrees C in complete growth medium. We propose that at least three (and possibly four) distinct types of pathways can act to reduce the levels of 365-nm radiation-induced strand breaks. A quantitative comparison of the number of breaks remaining with the number of lethal events remaining after repair in complete medium at 30 degrees C showed that between one and three breaks remain per lethal event in the wild-type and recA strains, whereas in the polA strain one order of magnitude more breaks were induced.