The number of single-strand breaks per single-strand genome produced by irradiating E. coli B/r in suspension has been measured as a function of elapsed time between irradiation and lysis. Between 0.2 and 2 sec after irradiation, the number of breaks in O2 and N2 remains constant at 32 and 9, respectively, following a 32 krad dose delivered in 12 msec. These values are the same at all temperatures between 0°C and 37°C. After 2 sec both excision and strand rejoining processes are observed with temperature-dependent rates. The oxygen enhancement ratio for ssb measured 300 sec after irradiation is 5.2 compared with 3.6 at 0.2 sec and 3.0 for cell survival. The radiosensitizer PNAP mimics oxygen in producing single-strand breaks, whereas NPPN does not affect the number of breaks observed in N2 but it does inhibit repair.

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