Levels of Ribonucleotide Reductase Activity during the Division Cycle of the L Cell

The levels of ribonucleotide reductase activity (measured as the conversion of 3H-cytosine monophosphate to 3H-dCMP) in cell-free extracts of partially synchronized cultures of L cells were measured as the cells moved through the division cycle. A strict parallelism was found between the measured enzyme levels and the fractions of the population that were forming DNA. This relationship between enzyme level and ability to replicate DNA indicated that the reductase activity fell rapidly when cells emerged from the DNA synthetic period, suggesting a lability of the enzyme in vivo. The lability in vivo was confirmed by measuring enzyme levels in cultures where further protein synthesis had been blocked with cycloheximide. Under these conditions, an apparent half-life of less than 2 hours was observed for ribonucleotide reductase in the L cell.