Recovery from vitamin B 12-induced unbalanced growth: Fate of the elevated ribonucleotide reductase activity in Euglena gracilis

Abstract

The fate of the elevated ribonucleotide reductase activity of vitamin B 12-deficient Euglena was determined in vitamin-replenished cells. Upon replenishment, the reductase activity was maintained at a constant value during resumption of DNA synthesis. With the completion of DNA synthesis the activity dropped over the post-DNA replicative period to a low but still abnormally high value. A peak of activity then appeared during the ensuing round of DNA synthesis. Addition of minimal of high doses of cycloheximide prevented these changes in the elevated reductase activity. These results suggested that the elevated activity found in deficient cells results from continued synthesis and accumulation of enzyme protein in the absence of the normally occurring degradation. At low and moderate concentrations of cycloheximide DNA synthesis was completed at slower rates than normal, while the high concentration permitted only a limited synthesis. Vitamin B 12-sufficient exponentially-growing cells maintained a constant level of activity. Addition of cycloheximide resulted in an exponential decay of enzyme activity by about 60%. A 50% drop in enzyme activity occurred within 1.2 h. A dose-dependent effect of cycloheximide on DNA synthesis was observed for these cells as well. The possible significance of this phenomenon is discussed.