Identification and functional analysis of the genes encoding Δ(6)-desaturase from Ribes nigrum†

Gamma-linolenic acid (γ-linolenic acid, GLA; C18:3 Δ(6, 9, 12)) belongs to the omega-6 family and exists primarily in several plant oils, such as evening primrose oil, blackcurrant oil, and borage oil. Δ(6)-desaturase is a key enzyme involved in the synthesis of GLA. There have been no previous reports on the genes encoding Δ(6)-desaturase in blackcurrant (Ribes nigrum L.). In this research, five nearly identical copies of Δ(6)-desaturase gene-like sequences, named RnD8A, RnD8B, RnD6C, RnD6D, and RnD6E, were isolated from blackcurrant. Heterologous expression in Saccharomyces cerevisiae and/or Arabidopsis thaliana confirmed that RnD6C/D/E were Δ(6)-desaturases that could use both α-linolenic acids (ALA; C18:3 Δ(9,12,15)) and linoleic acid (LA; C18:2 Δ(9,12)) precursors in vivo, whereas RnD8A/B were Δ(8)-sphlingolipid desaturases. Expression of GFP tagged with RnD6C/D/E showed that blackcurrant Δ(6)-desaturases were located in the mitochondrion (MIT) in yeast and the endoplasmic reticulum (ER) in tobacco. GC-MS results showed that blackcurrant accumulated GLA and octadecatetraenoic acids (OTA; C18:4 Δ(6,9,12,15)) mainly in seeds and a little in other organs and tissues. RT-PCR results showed that RnD6C and RnD6E were expressed in all the tissues at a low level, whereas RnD6D was expressed at a high level only in seeds, leading to the accumulation of GLA and OTA in seeds. This research provides new insights to our understanding of GLA synthesis and accumulation in plants and the evolutionary relationship of this class of desaturases, and new clues as to the amino acid determinants which define precise enzyme activity.