DNA SYNTHESIS IN SENSITIVE AND RESISTANT MUTANTS OF ESCHERICHIA-COLI B AFTER ULTRAVIOLET-IRRADIATION

The kinetics of growth (cell division) and of DNA-synthesis have been measured in Escherichia coli strains Bs-1, thy, B, thy and B/r, thy after UV irradiation (2537 Å). Using a new protocol for studying DNA-synthesis kinetics it has been shown that DNA synthesis is not permanently blocked by UV in the excision-deficient strain E. coli Bs-1, thy and that cell division proceeds without delay and at a normal rate in the survivors. In E. coli B/r, thy, DNA synthesis is stopped for various periods of time, increasing with the UV dose up to a maximum of about 0.75 generation time. After low doses of UV, synthesis resumes at near-normal rate for about 60 min and then changes to a rate proportional to the dose of UV. At higher doses, synthesis resumes at a rate proportional to the dose. Cell division is blocked in E. coli B/r, thy for aobut 120 min and then resumes at nearly the normal rate. The growth and DNA-synthesis kinetic data for E. coli B, thy are not as easy to interpret (because of ever-changing rates) as those for E. coli Bs-1, thy and B/r, thy, but are intermediate between these two extremes, consistent with its intermediate UV sensitivity. All 3 strains show a rapid increase in viability after UV irradiation during the first 30-60 min of growth in minimal medium as compared to zero-time plating on minimal-medium agar plates. This increase corresponds to a dose-reduction factor of 33% of E. coli Bs-1, thy and 69% for E. coli B/r, thy. © 1969.