The Association of Phosphatidylserine Synthetase with Ribosomes in Extracts of Escherichia coli

Abstract

Abstract The biosynthesis of phospholipids in Escherichia coli is catalyzed by enzymes most of which are associated with the bacterial membrane. In the present work, it has been found that the enzyme CDP-diglyceride: l-serine phosphatidyltransferase (phosphatidylserine synthetase) is exceptional in that it is not bound to membrane fragments in cell-free extracts of E. coli, but is predominantly associated with ribosomes. The enzyme resists extraction from ribosomes in buffers of high ionic strength. When ribosomes were allowed to dissociate in buffers containing low concentrations of magnesium, the enzyme was associated with both 50 and 30 S subunits. Phosphatidylserine is found only in trace amounts in E. coli, being rapidly converted to phosphatidylethanolamine by a decarboxylase which is a membrane-bound enzyme. Despite the apparently different intracellular localization of the two enzymes, their function in vivo must be tightly coupled. When the decarboxylase in living cells of E. coli was inhibited by the addition of hydroxylamine to the medium, phosphatidyl[1-14C]serine was shown to accumulate within 1 min. In the control cells, in which the decarboxylase was not inhibited, no labeled phosphatidylserine could be detected, showing that the decarboxylation of the newly formed phosphatidylserine must take place without detectable time lag.