Investigations on the association of phosphatidylserine synthase with the ribosomal component from Escherichia coli.

The CDP-1,2-diacyl-sn-glycerol (CDP-diacylglycerol):L-serine O-phosphatidyltransferase (EC 2.7.8.8, phosphatidylserine synthase) of Escherichia coli is the first enzyme in the pathway committed to the biosynthesis of the major lipid in E. coli, phosphatidylethanolamine. The enzyme is unique among the phospholipid biosynthetic enzymes due to its high affinity for ribosomes in crude extracts. We report here investigations which define the nature of this in vitro affinity for ribosomes. Phosphatidylserine synthase can be dissociated from ribosomes in the presence of various inorganic salts at high ionic strength. Dissociation was also brought about by cellular levels of the polyamine spermidine. These results suggest the interaction of the enzyme with ribosomes in vitro is primarily ionic in nature, and polyamines may prevent this interaction in vivo. In the presence of nonionic detergent-lipid substrate mixed micelles under assay conditions the enzyme is also dissociated from ribosomes. Dissociation does not occur in the presence of detergent alone or in the presence of lipids which are not substrates or products of the enzyme. This dissociation by lipid substrate indicates the enzyme is not associated with ribosomes during catalysis.