IRRADIATION OF PLASMID AND PHAGE DNA IN WATER-ALCOHOL MIXTURES - STRAND BREAKS AND LETHAL DAMAGE AS A FUNCTION OF SCAVENGER CONCENTRATION

We have measured the yields of strand break formation and biological i nactivation as a function of OH scavenger concentration for Co-60 gamm a-irradiated pBR322 plasmid and M13mp9 RF phage DNA. The yields of sin gle-strand breaks (ssbs), double-strand breaks formed proportionally t o dose (alphadsbs), and lethal damage (LD) decrease with increasing sc avenging capacity sigma, their ratios remaining approximately constant up to sigma is similar to 10(8) s-1. On a double-logarithmic plot the yields decrease linearly with sigma in parallel lines. At higher scav enging capacities, the yields, while still decreasing, level off to a different extent. Our results for the yields of ssbs and alphadsbs con firm those of Krisch et al. (1991) using SV40 DNA. The data were analy sed assuming that DNA damage is brought about by OH radicals, and a no n-scavengeable portion arising from the direct radiation effect. Using a model based on non-homogeneous scavenging kinetics, the dependence on scavenging capacity of the ssb yield could be quantitatively accoun ted for. From the scavenging dependence of the yield of dsbs which are formed quadratically with dose (betadsbs) and which are the result of two independent ssbs within a critical distance h, a value of about 1 3 basepairs was obtained for h. The parallel decrease in the yield of ssbs and alphadsbs with scavenging capacity was rationalized in terms of the Siddiqi-Bothe mechanism (Siddiqi and Bothe 1987). The efficienc y of this mechanism was found to be approximately 0.01. From the analy sis of the LD yields it was shown that up to sigma is similar to 10(8) s-1, inactivation is predominantly due to single OH radicals which le ad to LD with an efficiency of 0.12 per OH-induced ssb. At higher scav enging capacities, a non-scavengeable spur effect similar to the local ly multiply damaged sites mechanism of Ward (1988) mainly contributes to LD.