The second acylation reaction in glycerolipid biosynthesis is catalyzed by an sn-1-acylglycerol-3-phosphate acyltransferase. The enzyme of Limnanthes douglusii involved in triacylglycerol synthesis has an unusual specificity for very long chain acyl groups in both of its substrates, namely acyl-CoA and sn-1-acylglycerol-3-phosphate, and causes the enrichment of erucoyl groups in the sn-2 position of the seed oil of this plant species. We have isolated a cDNA clone encoding this embryo-specific, microsomal acyltransferase via heterologous complementation of an Escherichia coli mutant deficient in sn -1-acyl-glycerol-3-phosphate acyltransferase activity. The open reading frame of the cDNA insert encodes a protein with a length of 281 amino acids, with three predicted membrane-spanning domains and of about 31.7 kDa. The sequence exhibits substantial sequence similaritiy to the sn-1-acylglycerol-3-phosphate acyltransferase of E. coli. The corresponding transcript was detectable in developing embryos but not in leaves of L. douglasii, and expression of the open reading frame in E. coli caused sn-1-acylglycerol-3-phosphate acyltransferase activity which showed properties different from those of the bacterial acyltransferase but typical of the L. douglusii enzyme involved in triacylglycerol biosynthesis.