Regulation of phosphatidate phosphatase (EC 3.1.34) activity was examined in Saccharomyces cerevisiae cells supplemented with phospholipid precursors. Addition of inositol to the growth medium of wild-type cells resulted in a twofold increase in phosphatidate phosphatase activity. The increase in phosphatidate phosphatase activity was not due to soluble effector molecules, and inositol did not have a direct effect on enzyme activity. The phosphatidate phosphatase activity associated with the mitochondrial, microsoinal, and cytosolic fractions of the cell was regulated by inositol in the same manner. Cells supplemented with inositol had elevated phospholipid levels and reduced triacylglycerol levels compared with unsupplemented cells. Serine, ethanolamine, and choline did not significantly affect the phosphatidate phosphatase activity of cells grown in the absence or presence of inositol. Enzyme activity was not regulated in inositol biosynthesis regulatory mutants, suggesting that regulation by inositol is coupled to regulation of inositol biosynthesis. Phosphatidate phosphatase activity was pleiotropically expressed in structural gene mutants defective in phospholipid biosynthesis. These results suggested that phosphatidate phosphatase was regulated by inositol at a genetic level.Phosphatidate (PA) is an important branch point intermediate for the synthesis of phospholipids, diacylglycerols, and triacylglycerols in the unicellular eucaryote Saccharomyces cerevisiae (19). In the primary pathway of phospholipid biosynthesis, phosphatidylcholine is derived from PA via the reaction sequence PA -+ CDP-diacylglycerol phosphatidylserine -> phosphatidylethanolamine -* phosphatidylmonomethylethanolamine -* phosphatidyldimethylethanolamine -> phosphatidylcholine (19). Phosphatidylinositol, phosphatidylglycerol, and cardiolipin are also derived from CDP-diacylglycerol (19). Diacylglycerols and triacylglycerols are derived from PA via the reaction sequence PA -* diacylglycerol --triacylglycerol (19). The diacylglycerol synthesized from PA can also be used for synthesis of phosphatidylethanolamine and phosphatidylcholine by the auxiliary CDP-ethanolamine-and CDP-choline-based pathways, respectively (28), when synthesis of phosphatidylserine is blocked and cells are supplemented with ethanolamine and choline (2,32,36). Addition of.ethanolamine or choline to inositol-containing growth medium leads to repression of the enzymes (CDPdiacylglycerol synthase [24], phosphatidylserine,synthase [3,7,30,37], phosphatidyiserine decarboxylase [8], and the phospholipid N-methyltransferases [8,30,42,43,45,46]) in the primary pathway for phosphatidylcholine biosynthesis. Repression of these enzymes by ethanolamine and choline is absolutely dependent on inositol (3,24,30,37,45,46). Inositol alone partially represses these enzymes (3,24,30,37,45,46). The enzyme responsible for synthesis of phosphatidylinositol from CDP-diacylglycerol (phosphatidylinositol synthase) is not repressed by phospholipid precursors (15,30). However, the enzyme resp...

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