The Effect of Oxygen on Mutation Induction by X-Rays

Abstract

A purineless mutant and a streptomycin-dependent mutant of Escherichia coli strain B were used for detn. of alterations in the induction of mutations by X-radiation through changes in O2 tension. Results showed that a high O2 tension increased the sensitivity of both strains to lethal effects of X-rays and the mutation rates of both were increased with length of exposure of X-rays in either O2 or N2; also, in the case of the purine-free strain only one mechanism is involved in both lethal and specific genetic effects, whereas in the streptomycin-dependent strain there are different mechanisms. Cells were washed twice in [image]/15 phosphate buffer at pH 6.9 with resuspension in the buffer to give 2.0-3.5x109 cells per ml. One-ml. amts. were placed in 2-ml. thin-walled volumetric tubes fitted with ground glass stoppers which carried bubbling and exhaust tubes equipped with stopcocks, and arrangement by which air could be removed and replaced by the desired gas at a slightly positive pressure. Suspensions were bubbled for 15-20 min. and immersed in 2[degree]C just prior to irradiation. By the use of radial holes in a lucite ring-shaped holder and a 250-thimble ionization chamber of a standardized Victoreen dosimeter the tubes were exposed in a predetermined uniform field of radiation and the exposure rate in the exact area measured. The X-ray source was a General Electric Maximar Model, 250-kvp. unit operated at 250-kvp. and 15 ma. The inherent filtration was equivalent to 3mm. of Al and exposures were at a constant rate of 40 kr per hr. at target distance of 25 cm.