Effects of Temperature on Spontaneous and Induced Mutations in Escherichia Coli

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Abstract
Strain B/r of E. coli was grown in a synthetic medium "A" with aeration for 18-24 hrs., diluted to approx. 108 cells per ml. and exposed to 800 ergs per mm2 of u.-v. radiation at room temp. Irradiated suspensions were incubated at 37[degree], 25[degree] or 16[degree]C. At time intervals based on previously established growth curves ranging from 0 to 14 generations, plates were removed and chilled rapidly to arrest growth. Some plates were sprayed with bacteriophage T1 to determine the no. of bacteriophage-resistant clones. On nutrient agar plates the generation time of untreated and irradiated bacteria is 20 min. at 37[degree], 1 hr. at 25[degree], and 3 hrs. 20 min. at 16[degree]. The lag phase for untreated cells is 1 hr. 10 min. at 37[degree], 3 hrs. 30 min. at 25[degree] and 10 hrs. 40 min. at 16[degree], and for irradiated bacteria 3 hrs, 10 hrs. and 30 hrs.,resp. When the number of T1 resistant clones per 108 non-irradiated cells plated is plotted against generation time at the 3 temps., the 3 curves are identical, indicating that the spontaneous mutation rate has the same response to temp. and to generation time. When induced mutation rate is plotted against post-irradiation generations at the 3 temps. and corrected for spontaneous mutation rate, the final yield of induced mutations at 16[degree] is 5% of that at 37[degree], with little, if any, difference between yields at 25[degree] and 37[degree]. The % of the total yield of induced mutations per post-irradiation generation is the same at 37[degree] and 16[degree], indicating that the pattern of delayed appearance is stable for a particular mutation. In order to test the temp.-sensitive period, irradiated bacteria were incubated at 37[degree] for one generation, then transferred to 16[degree] for the remaining divisions. In reciprocal tests the first post-irradiation division takes place at 16[degree] and the remainder at 37[degree]. The results indicate that the yield of induced mutations depends on the temp. during the first post-irradiation generation. When irradiated cells are exposed to one temp. for various fractions of the 1st generation period, intermediate yields are obtained, and the effect of the initial temp. is proportional to the fraction of the 1st generation passed under its influence. The temp. coefficients for spontaneous mutation per unit time, cell division, and delayed appearance of induced mutations per unit time are identical (Q10 = ca.3), indicating an association among the 3 processes. The effect of post-treatment incubation at 16[degree] can be duplicated at 37[degree] by reducing the u.-v. dose from 800 to 100 ergs per mm2.

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