Responses to Accelerated Heavy Ions of Spores of Bacillus subtilis of Different Repair Capacity

Abstract

Inactivation, mutagenesis to histidine prototrophy and the involvement of DNA repair were studied in spores of Bacillus subtilis irradiated with heavy ions at LBL, Berkeley and GSI, Darmstadt. Five groups of ions were used varying in energy up to 18.6 MeV/u. Action cross sections of both inactivation and mutagenesis show a similar dependence on ion mass and energy: for lighter ions, the lethal response is nearly energy-independent or decreasing with energy; these light ions, up to 18.6 MeV/u, induce hardly any mutations. For heavier ions, the lethal as well as the mutagenic responses increase with ion mass and energy up to a maximum or saturation. The efficiency of DNA repair to improve survival and the mutagenic efficiency per lethal event, both, increase with ion energy up to a saturation value which, depending on strain and endpoint, either roughly coincide with the X-ray value or is smaller than that after X-ray treatment. At energies of 1 MeV/u or below, neither repair nor mutations appear to be involved. The results support previous suggestions on the importance on the importance of the radial distribution of the energy around the ion track in biological action cross section and the evidence that the entire core of the spore represents the sensitive site in responses to heavy ions.