DNA synthesis inEscherichia coli B/r after UV-irradiation

When studying the kinetics of DNA synthesis, growth and cell division inEscherichia coli B/r after irradiation with different doses of UV-radiation (254 nm) we could demonstrate, by means of pulse incorporation of³H-thymidine, a lag in DNA synthesis after the irradiation. The relative rate of the restored DNA synthesis (related to the number of viable cells) was higher than in the non-irradiated culture. After 3 h the rate of DNA synthesis settled at a constant value, which was identical with the control rate up to the “critical dose” of 20 J/m². The irradiated cell population is heterogenous and contains basically two categories of cells — surviving and non-surviving. Cells of both types contribute to DNA synthesis restored after the lag period to a different extent. During the first hour after the irradiation even the nonviable portion of the population,i.e. cells that do not form colonies but are still penicillin-sensitive, is involved in the DNA synthesis.