Enhanced Survival of Gamma-irradiated Escherichia Coli Following Pretreatment with Dithiothreitol

Summary

Survival of three strains of Escherichia coli K12 was studied with respect to radiation protection by dithiothreitol (DTT). The three strains compared were AB2462 recA, AB2470 rec21 and their DNA repair-competent prototype, AB1157. The strains were incubated in 10 mmol dm⁻³ DTT for 60 min and allowed an expression period for SOS functions to appear which may have been induced by DTT. Following the expression period the DTT-incubated cells and incubated control cells were irradiated. When AB1157 cells were pretreated with chloramphenicol (200 µg cm⁻³) for a period of 30 min prior to addition of the induction media no increase in survival was seen. When catalase (0·1 mg cm⁻³) was added to the AB1157 cells prior to the induction media a decrease in the degree of induction was noted with an enhancement ratio (ER) of 0·893 (ER⁻¹ = 1·12). Furthermore, DTT-treated AB2462 and AB2470 demonstrated no increase in survival when compared to control cells. In radiation experiments on either strain of E. coli with or without DTT present during irradiation, the following were observed: (1) survival of AB1157 was enhanced with a dose modification factor (DMF) of 1·7 with DTT present and 1·3 with pretreatment; (2) the rec mutants showed no change in survival at any dose with a DMF of approximately 1·0. Results indicate that, using our protocol, inducible repair is of more importance than free radical scavenging by DTT. Furthermore, DTT-treated AB2462 demonstrated no increase in survival when compared to control cells. In radiation experiments on either strain of E. coli with and without DTT present during irradiation, the following were observed: (1) survival of AB1157 was enhanced with a DMF of 1·7 with DTT present during irradiation and 1·3 with only pretreatment; (2) the recA and recB mutants showed no change in cell survival at any dose with a DMF of approximately 1·0. Results indicate that, using our pretreatment protocol, inducible repair is of more importance in protection than free radical scavenging by DTT.