| Abstract: | Two distinct pathways for the incorporation of exogenous fatty acids into phospholipids were identified in Escherichia coli. The predominant route originates with the activation of fatty acids by acyl-CoA synthetase followed by the distribution of the acyl moieties into all phospholipid classes via the sn-glycerol-3-phosphate acyltransferase reaction. This pathway was blocked in mutants (fadD) lacking acyl-CoA synthetase activity. In fadD strains, exogenous fatty acids were introduced exclusively into the 1-position of phosphatidylethanolamine. This secondary route is related to 1-position fatty acid turnover in phosphatidylethanolamine and proceeds via the acyl-acyl carrier protein synthetase/2-acylglycerophosphoethanolamine acyltransferase system. The turnover pathway exhibited a preference for saturated fatty acids, whereas the acyl-CoA synthetase-dependent pathway was less discriminating. Both pathways were inhibited in mutants (fadL) lacking the fatty acid permease, demonstrating that the fadL gene product translocates exogenous fatty acids to an intracellular pool accessible to both synthetases. These data demonstrate that acyl-CoA synthetase is not required for fatty acid transport in E. coli and that the metabolism of exogenous fatty acids is segregated from the metabolism of acyl-acyl carrier proteins derived from fatty acid biosynthesis. |
