Phases of thymineless death inEscherichia coli 15 TAU

On the basis of the course of loss of colony-forming ability it was possible to disting guish at least three phases of thymineless death in a culture ofEscherichia coli 15 TAU starved in a thymine-free medium enriched with arginine and uracil. The main differences between the individual phases were: (a) half-time of the loss of colony-forming ability; (b) only in the first phase was thymineless death reversible; (c) beginning in the second phase, part of the cells lysed after thymine was added to the liquid medium or when cells were plated on agar with complete medium; (d) lysis was much slower in the third phase than in the second; (e) in the presence of caffeine the course of the first and second phase was not affected but cells did not die in the third phase. Cells surviving in the third phase in the presence of caffeine are probably those which did not die even in medium T^?A^?U^? (Maaløe & Hanawalt, 1961). Transition from one phase to another was caused neither by change in the composition of medium during thymine starvation nor by heterogeneity of this culture from a genetical point of view.