Participation of Soluble Phophatidylserine Synthetase in Phosphatidylethanolamine Biosynthesis in Escherichia coli Membrane

Phosphatidylserine synthetase occurs in a soluble and a ribosome‐bound form in Escherichia coli. The solble enzyme of E. coli B was partially purified by DEAE‐Sephadex chromatography. The ribosomal enzyme was obtained by differential centrifugation. Two types of the enzyme had the same Km (150 μM) for l‐serine. The soluble enzyme was required for the biosynthesis of phosphatidylethanolamine from phosphatidic acid via CDP‐diglyceride in the membrane. The ribosomal enzyme was less effective to the phosphatidylethanolamine formation than the soluble enzyme. By the sucrose‐density‐gradient centrifugation method the soluble enzyme was proved to be loosely associated with the membrane.