Response of Escherichia coli to ethyl methanesulfonate: Influence of growth phase and repair ability on survival and mutagenesis

The effects of altering the cell growth rate (physiological state) and DNA repair capacity (genetic state) on susceptibility to inactivation and mutagenesis by ethyl methanesulfonate (EMS) were studied in 4 strains of E. coli. Logarithmic and stationary phase cells of the polymerase I deficient mutant, P₃₄₇8 polA, a recombination deficient mutant, DZ₄₁₇ recA, and the respective parental strains, W₃₁₁₀pol⁺ and AB₂₅₃ rec⁺, were exposed to EMS and the surviving fraction and mutant frequency determined. At the same EMS concentration both mutants were more susceptible to inactivation than the parental strains. In all 4 strains, log phase cells were more sensitive to inactivation than stationary cells. The surviving fraction of stationary cells exceeded log cells by a factor of 18 for polA, 6 for recA, and about 2 for the parental strains. In all strains, except recA, log phase cells exhibited higher spontaneous mutant frequencies than stationary phase cells. At the same concentration of EMS, survivors of both polA and recA showed more than 10-fold higher induced frequencies than the wild types. However, at the same survival levels the repair deficient mutants exhibited induced mutant frequencies comparable to the repair proficient strains. There was no significant effect of growth phase on EMS induced mutability in recA or the parental strains. In marked contrast, the polymerase I deficient mutant shows both a higher spontaneous frequency and a greater than 10-fold higher EMS induced mutant frequency in log phase cultures compared to stationary phase cultures. Our results support the hypothesis that cellular susceptibility to alkylating agents is influenced by both the genetic capability for repair and the particular physiological state of the cell.