Formation of thymidine kinase and deoxycytidylate deaminase in synchronized cultures of chinese hamster cells temperature‐sensitive for DNA synthesis

Cytosol thymidine kinase (TK) and deoxycytidylate (dCMP) deaminase formation was investigated in synchronized cultures of K12 Chinese hamster cells which have a temperature‐sensitive lesion affecting the initiation of DNA synthesis. Enzyme formation was found to be cycloheximide‐sensitive and also temperature‐dependent. Beginning at about six hours after addition of medium with 10% calf serum to serum‐depleted K12 cultures, cytosol TK and dCMP deaminase activities increased when the cultures were incubated at 36.5°, but not at 40.5°. When cultures were shifted from 36.5° to 40.5° at 4, 6, or 8 hours after serum addition, TK activity continued to increase, though not to the level observed at ten hours in cultures maintained at 36.5°. Actinomycin D addition at the time of serum reversal or four hours later blocked the TK increase normally observed at the permissive temperature at ten hours. However, when actinomycin D addition was delayed for six or eight hours after serum addition, the increase in TK measured at ten hours resembled that observed in the temperature shift‐up experiments. The results provide evidence that the mutation in K12 Chinese hamster cells most likely blocks the progression through G1 into S and suggest that transcription or post‐transcriptional processing required for TK formation is affected.