Radiation sensitization of Escherichia coli B/r by 2'-chloro-2'-deoxythymidine under various irradiation conditions

In order to elucidate the mechanism of sensitization of E. coli B/r cells to X-irradiation by 2'-chloro-2'-deoxythymidine (2'Cl-TdR), the survival curves of the cells in which 2'Cl-TdR was incorporated into DNA were obtained following X-irradiation under various conditions. The marked sensitization of E. coli cells by 2'Cl-TdR to the killing action of X-rays was observed, when E. coli cells labelled with 2'Cl-TdR were exposed to X-rays in the absence of oxygen as well as in the presence of oxygen. The sensitization factor calculated from inactivation constants from survival curves irradiated in the absence of O2 was about a half of that obtained in the presence of O2. Under the conditions where 2'Cl-TdR was not incorporated into the DNA of E. coli cells, the presence of 2'Cl-TdR in the cell suspension fluid at the time of irradiation caused no sensitization of the cells to X-irradiation. The sensitization factor for 2'Cl-TdR obtained under N2O was almost same as that obtained under N2. It was also observed that the sensitization factor obtained in the presence of glycerol at a concentration of 1 mol dm-3 under N2 was similar to that obtained in the absence of glycerol. These results indicated that the direct effect of ionizing radiation on DNA was closely associated with the sensitization of E. coli B/r cells by 2'Cl-TdR and that the radical at the C-2' position of the deoxyribose moiety in DNA produced by X-irradiation was transformed into lethal damage for E. coli cells even in the absence of O2. However, this transformation occurred more efficiently in the presence of O2 than in the absence of O2.