Modifications of the Inactivation by Ionizing Radiations of the Transforming Activity of DNA in Spores and Dry Cells

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Abstract
Both the colony-forming ability of spores of Bacillus subtilis and the transforming ability of DNA extracted from them after irradiation are about four times as resistant to ionizing radiation as are the corresponding activities for dry vegetative cells. The radiation sensitivity of the colony-forming ability of irradiated subtilis spores is reduced by about a factor of 2 by a heat treatment after irradiation, as previously found, but the transforming activity of extracted DNA is unaffected by such a heat treatment. Such thermorestoration occurs only for spores of intermediate humidity, vanishing for spores that are very dry or quite damp. This is an agreement with the data of others. There is no effect on colony-forming ability of heat treatment after irradiation on irradiated spores that have been sporulated in a medium containing 5-bromo-deoxyuridine. These spores show the same radiosensitivity as spores formed in normal media. No effect of a heat treatment after irradiation could be demonstrated on the colony-forming ability of dry vegetative B. subtilis or yeast cells, on the transforming ability of DNA irradiated in dry pneumococcus or vegetative B. subtilis cells, or on the plaque-forming ability of dry T-l bacteriophage.