High-Level Production of γ-Linolenic Acid in Brassica juncea Using a Δ6 Desaturase from Pythium irregulare

摘要

γ-Linolenic acid (GLA), a nutritionally important fatty acid in mammals, is synthesized by a Δ6 desaturase. Here, we report identification of PiD6, a new cDNA from the oleaginous fungus, Pythium irregulare, encoding a 459-amino acid protein that shares sequence similarity to carboxyl-directed desaturases from various species. Expression of PiD6 in yeast (Saccharomyces cerevisiae) revealed that it converts exogenously supplied linoleic acid into GLA, indicating that it encodes a Δ6 fatty acid desaturase. Expression of the desaturase in Brassica juncea under the control of the Brassica napus napin promoter resulted in production of three Δ6 unsaturated fatty acids (18:2–6, 9; 18:3–6, 9, 12; and 18:4–6, 9, 12, 15) in seeds. Among them, GLA (18:3–6, 9, 12) is the most abundant and accounts for up to 40% of the total seed fatty acids. Lipid class and positional analysis indicated that GLA is almost exclusively incorporated into triacylglycerol (98.5%) with only trace amounts found in the other lipids. Within triacylglycerols, GLA is more abundant at the sn-2 position.