Breakdown of DNA in x-irradiated Escherichia coli.

A comparison of differences in incorporation and loss of radio-activity between two strains of Escherichia coli shows that: (a) three times as much irradiation is necessary to produce the same reduction in incorporation of H(3)-thymidine in B/r, the resistant strain, as in B(s - 1), the sensitive one; (b) radioactivity is lost from the DNA of previously labeled bacteria during the first few cell generations after X-ray exposure, and even though the initial rate of loss is similar for all strains, the sensitive one loses much more label; (c) loss of DNA is a complicated function of dose. Losses increase with dose up to 25 or 50 kr in both strains; with higher doses, losses decrease in B(s - 1) but are unchanged in B/r. Since in both strains labeled RNA is retained in irradiated cells, lysis has not occurred but the DNA is broken down into small pieces which leak from each cell. Losses from either strain do not occur at ice-bath temperature, indicating that breakdown is a function of metabolic processes. A proposed mechanism for X-ray damage and repair is advanced.