Competition between Tetramethylpiperidinol N-Oxyl and Oxygen in Effects on Single-Strand Breaks in Episomal DNA and in Killing after X-Irradiation in Escherichia coli

Abstract

The x-ray sensitivity of E. coli K12 recA under various concentrations of N-oxyls and oxygen was determined from the survival of colony-forming ability. Oxygen sensitized these cells 1.8 times while tetramethylpiperidinol N-oxyl (TMPN) in the absence of oxygen sensitized them 4.6 times. When present together the marked sensitization by TMPN at 5 mM was competitively reversed by oxygen, one oxygen molecule being as effective as 37 molecules of TMPN. When the level of endogenous sulfhydryls was lowered by pretreating the cells with N-ethylmaleimide, 1/4.8 of the concentration of TMPN was sufficient to raise the x-ray sensitivity. The yield of x-ray induced breaks in circular phage λ DNA molecules in λ lysogens was also measured under various experimental conditions. TMPN neither increased the frequency of DNA single-strand breaks under anoxia nor prevented oxygen from promoting breaks in the DNA. These results show that radiation-induced reactive intermediates or transients, presumably free radicals, respon...