Transcription of the CTP:Phosphocholine Cytidylyltransferase Gene Is Enhanced during the S Phase of the Cell Cycle*

We have studied the transcription of the CTP:phosphocholine cytidylyltransferase (CT ) gene in C3H10T1/2 fibroblasts as a function of the cell cycle. The cells were incubated for 48 h with 0.5% fetal bovine serum. The cells were induced into the G1 phase of the cell cycle by the addition of medium with 10% fetal bovine serum. The cells began the synthesis of DNA after 12 h. At 16 and 20 h there was an increased amount of CT mRNA that coincided with an increase in the expression of CT proximal promoter-luciferase constructs ( 201/ 38 and 130/ 38). Luciferase constructs with the basal promoter ( 52/ 38) showed no change in activity during the cell cycle. Incorporation of [H]choline into phosphatidylcholine began to increase by 8 h after the addition of serum and peaked at 18 h. The mass of phosphatidylcholine nearly doubled between 8 and 26 h after addition of serum. CT activity increased by 6 h after serum addition and was maintained until 22 h. Thus, the increase of phosphatidylcholine biosynthesis in the G1 phase of the cell cycle is not due to enhanced transcription of the CT gene. Instead increased transcription of the CT gene occurred during the S phase of the cell cycle in preparation for mitosis.