REGULATION OF PHOSPHOLIPID BIOSYNTHESIS IN SACCHAROMYCES-CEREVISIAE BY CTP

In the yeast Saccharomyces cerevisiae, the major membrane phospholipid phosphatidylcholine is synthesized by the CDP-diacylglycerol and CDP- choline pathways. We examined the regulation of phosphatidylcholine sy nthesis by CTP. The cellular concentration of CTP was elevated (2.4-fo ld) by overexpressing CTP synthetase, the enzyme responsible for the s ynthesis of CTP. The overexpression of CTP synthetase resulted in a a- fold increase in the utilization of the CDP-choline pathway for phosph atidylcholine synthesis. The increase in CDP-choline pathway usage was not due to an increase in the expression of any of the enzymes in thi s pathway. CDP-choline, the product of the phosphocholine cytidylyltra nsferase reaction, was the limiting intermediate in the CDP-choline pa thway. The apparent K-m of CTP (1.4 mM) for phosphocholine cytidylyltr ansferase was 2-fold higher than the cellular concentration of CTP (0. 7 mM) in control cells. This provided an explanation of why the overex pression of CTP synthetase caused an increase in the cellular concentr ation of CDP-choline, Phosphatidylserine synthase activity was reduced in cells overexpressing CTP synthetase. This was not due to a transcr iptional repression mechanism, Instead, the decrease in phosphatidylse rine synthase activity was due, at least in part, to a direct inhibiti on of activity by CTP. These results show that CTP plays a role in the regulation of the pathways by which phosphatidylcholine is synthesize d. This regulation includes the supply of CTP for the phosphocholine c ytidylyltransferase reaction in the CDP-choline pathway and the inhibi tion of the phosphatidylserine synthase reaction in the CDP-diacylglyc erol pathway.