The ability in vitro of yeast mitochondrial and microsomal fractions to synthesize lipid de novo was measured. The major phospholipids synthesized from sn-[2-3H]glycerol 3-phosphate by the two microsomal fractions were phosphatidylserine, phosphatidylinositol and phosphatidic acid. The mitochondrial fraction, which had a higher specific activity for total glycerolipid synthesis, synthesized phosphatidylglycerol, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine and phosphatidic acid, together with smaller amounts of neutral lipids and diphosphatidylglycerol. Phosphatidylcholine synthesis from both S-adenosyl[Me-14C]methionine and CDP-[Me-14C]choline appeared to be localized in the microsomal fraction.
