Isolation of a novel C18-Δ9 polyunsaturated fatty acid specific elongase gene from DHA-producing Isochrysis galbana H29 and its use for the reconstitution of the alternative Δ8 pathway in Saccharomyces cerevisiae.

A novel gene (IgASE2) encoding a C18-Δ9 polyunsaturated fatty acids specific (C18-Δ9-PUFAs-specific) elongase was isolated and characterized from DHA-rich microalga, Isochrysis galbana H29. The IgASE2 gene was 1,653 bp in length, contained a 786 bp ORF encoding a protein of 261 amino acids that shared 87% identity with Δ9 elongase, IgASE1, and possessed a 44 bp 5'-untranslated region (5'-UTR) and a 823 bp 3'-untranslated region (3'-UTR). IgASE2, by its heterologous expression in Saccharomyces cerevisiae, elongated linoleic acid (LA, 18:2n-6) and α-linolenic (ALA, 18:3n-3) to eicosadienoic acid (EDA, 20:2n-6) and eicosatrienoic acid (ETrA, 20:3n-3), respectively. The conversions of LA to EDA and ALA to ETrA were 57.6 and 56.1%, respectively. Co-expression of this elongase with Δ8 desaturase required for the synthesis of C20-polyunsaturated fatty acids resulted in the accumulation of dihomo-γ-linolenic acid (20:3n-6) from LA and eicosatetraenoic acid (20:4n-6) from ALA. These results demonstrated that IgASE2 exhibited C18-Δ9-PUFAs-specific elongase activity and the alternative Δ8 pathway was reconstituted.