Base substitution spectra of nalidixylate resistant mutations induced by monochromatic soft X and Co-60 gamma-rays in Bacillus subtilis spores

Bacillus subtilis spores were exposed to three types of photons, monochroma tic soft X-rays with the energy corresponding to the absorption peak of pho sphorus K-shell electron (2,153 eV) and with the slightly lower energy (2,1 47 eV), and Co-60 gamma-rays. From the irradiated spores, 233 mutants exhib iting nalidixic acid resistance were isolated, and together with 94 spontan eous mutants, the sequence changes in the 5'-terminal region of the gyrA ge ne coding for DNA gyrase subunit A were determined. Among eighteen alleles of the gyrA mutations, eight were single-base substitutions, nine were tand em double-base substitutions, and one was a double substitution skipping a middle base pair. About 6% of the radiation-induced mutations were tandem d ouble-base substitutions, whereas none was observed among the spontaneous o nes. Among spontaneous mutations, A:T and G:C pairs were equally subjected to mutations, whereas the substitutions from G:C pairs and those to A:T pai rs predominated among those induced with soft X-rays. The peak-energy Xrays were more effective in killing and causing mutations than the low-energy X -rays, however, there seemed no base-change events uniquely attributable to phosphorus K-shell absorption.