The relation between survival and the initial yield of DNA strand breaks for E coli B/r

The yield of single-strand breaks (SSB) present 200 msec. after irradiation of E coli B/r has been measured under various experimental conditions. Experiments were performed either under oxic or anoxic conditions and in the presence of a dose-modifying compound. The compounds chosen were: dimethyl sulphoxide, cysteine, glycerol and parachloromercuribenzoate which have the properties of modifying the radiation response in both oxic and anoxic conditions. The survival curve parameters of the B/r were also measured under corresponding conditions to the SSB. The results show that the slope of the survival curves are related to the corresponding yield of SSB by the simple relationship: K=Ax + B where x is the yield of SSB per krad and A and B are constants. A was found to have the values 1.7 x 10-2 and 5.7 x 10-3 under oxic and anoxic conditions respectively whereas the corresponding values of B were found to be 3.0 x 10-2 krad-1 and 2.3 x 10-2 krad-1. At high concentrations of DMSO the yields of SSB per krad reach limiting values of 1.0 and 0.5 in oxic and anoxic conditions respectively. These limiting values were interpreted as a measure of the damage produced by the direct absorption of energy in the DNA and its modification in the presence of oxygen. The results suggest that the survival of irradiated E coli B/r depends upon at least two classes of effect, one of which is in a fixed relation to the initial yield of SSB