Single- and double-strand breaks of DNA in electron-irradiated Bacillus subtilis spores were measured and compared with those in irradiated vegetative cells by zone sedimentation in alkaline and neutral pH sucrose gradients. DNA in spores was much more resistant to both types of damage than DNA in vegetative cells. Induction of one single-strand break in DNA of spore genome required 2.8 krad compared to 0.13 krad in vegetative cell genome. In other terms, energies of 857 eV in spore DNA and 41 eV in vegetative cell DNA were required to induce one single-strand break, calculated from molecular weights given by weight-average sedimentation distances; or 835 eV and 46 eV, calculated from weight-average molecular weights. The resistance of DNA was maintained after spores were converted to osmotically sensitive spheroplasts; however, the resistance was lost by rupture of the spheroplast structure. This resistance of DNA was not due to the repair activity of spores nor to protective substance in spore coat and cortex. Structural integrity of DNA is considered to be most important in protection of DNA in spores. According to calculations based on the target theory, the idea that one double-strand break in DNA of genome corresponds to one lethal hit seems to be valid in spores and vegetative cells.

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