In a
thymine-deprived culture, the mutant cells (deficient in
dTDP-glucose pyrophosphorylase activity and named Ter-15) lose viability at a faster rate, form longer filaments for the first 60 min and lose
thymidine nucleotides and dTDP-
sugar pools at a faster rate for the first 15 min than those of the parent cells, but the dTDP-
sugar pool in the parent cells is maintained at high concentration for the first 90 min during
thymine starvation. In the recovery of cell growth after re-addition of
thymine into the
thymine-deprived culture, parent cells recommence growth immediately, but the mutant cells (Ter-15) show a lag-phase for 45 min after which time their growth recommences. The rate of
dTTP synthesis for the first 10 to 15 min after re-addition of
thymine to
thymine-deprived cultures of parent and mutant (Ter-15) cells is three-fold higher than that of
thymine nondeprived culture (control), but the rates of
dTMP and dTDP-
sugar syntheses are the same as those of the control. The total
DNA synthesis after re-addition of
thymine is equal to that of the control, and the period of
thymine starvation other than the number of viable cells during
thymine starvation plays an important role. After separation of the filament cells from normal-sized cells by
sucrose gradient centrifugation, the initial rate of
DNA synthesis of filament cells is three-fold faster than that of normal-sized cells. These results show that the dependency of
DNA synthesis upon
dTTP concentration is maintained after re-addition of
thymine into
thymine-deprived culture.