The energy dependence of mutations produced in Escherichia coli by low doses of x-radiation

Abstract

Resistance to the proline analogue, azetidine-2-carboxylic acid, was used as a selection technique with E. coli in order to measure the production of mutations caused by low doses of X-radiation. An improved procedure was required to measure the small changes in the mutation rate. Since the Coulter Counter provided a precise estimate of bacterial numbers, it was found to be essential in this procedure. In addition, a system for producing K X-rays and highly filtered X-rays was developed. This system produced X-rays of good homogeneity whose quality could easily be defined. Bacterial cultures received exposures from 10 R to 138 R at various energies from 29 keV to 88 keV. X-ray exposure at the 29 keV energy produced a significant increase in the bacterial mutation rate. Exposure at 37 keV and 58 keV caused a significant decrease in the bacterial mutation rate. Bacterial cultures showing logarithmic growth were much more sensitive to irradiation than cultures in a non-growing state. Mutants were characterized by reversion with the alkylating agent ethyl methane sulfonate (EMS) and the less specific, but more potent, agent N-methyl-N'-nitro-N-nitrosoguanidine (NTG). EMS reverted the spontaneously occurring mutants and those produced by exposure to 29 keV X-rays more readily than mutants produced by exposure to higher energy X-rays. No correlation was observed with NTG reversion. The data in these experiments indicate that the rate and type of mutations produced by low doses of X-rays were energy dependent.