Deoxycytidylate Synthesis and Entry into the Period of Deoxyribonucleic Acid Replication in Rabbit Kidney Cells

With kidney cells cultured directly from the animal, increases in the labeling of the deoxycytidylate pool from 3H-cytidine and in deoxyribonucleic acid polymerase activity begin at about the time of initiation of DNA formation. Some of the possible causal interrelationships of these three changes have been examined. With the use of phleomycin to block DNA synthesis, it has been shown that neither the increase in dCMP nor the increase in polymerase activity is triggered by the act of DNA replication. Hydroxyurea reversibly blocks the rise in the labeling of the dCMP pool. With this inhibitor, it has been shown that the normal increase in the deoxyribonucleotide pool is not essential for the increase in DNA polymerase activity. Nor does the rise in the deoxyribonucleotide pool appear to be required for entry of a kidney cell into the period of DNA synthesis. When cells inhibited with hydroxyurea were irradiated with x-rays (which prevent entry into the replicative stage but have little effect upon the continuation of DNA formation), removal of the chemical inhibitor was followed by the synthesis of DNA.