The Modification of the Yields of Single Strand Breaks in DNA of Gamma-Irradiated Escherichia coli

Abstract

The yield of single strand breaks in DNA in the γ-irradiation of Escherichia coli $15 {\rm T}^{-}$, as measured by the alkaline sucrose gradient technique, is enhanced by two enzyme inactivating agents, namely, sodium cyanide and parachloromercuribenzoate (PCMB), but to different extents. The G-value of 0.26 obtained in air for the yield of single strand breaks in the absence of additives is increased to 1.4 by sodium cyanide and to 5.4 by PCMB. In nitrogen, the yield for sodium cyanide-treated cells is 0.55 and that for PCMB-treated cells 5.3. Irradiation of PCMB-treated cells in the presence of histidine gives a lower yield of 1.0. β-Mercaptoethylamine (MEA) completely neutralizes the enhancing effect of PCMB, the residual yield being 0.27. For cyanide-treated cells, both histidine and MEA give the same limiting yield of 0.45. The cell is apparently capable of rejoining by itself breaks attributable to free radical attack as well as some produced by the direct action of radiation. These results indicat...