The moss Physcomitrella patens contains high levels of arachidonic acid and lesser amounts of eicosapentaenoic acid. Here we report the identification and characterization of a ⌬ 5 -desaturase from P. patens that is associated with the synthesis of these fatty acids. A full-length cDNA for this desaturase was identified by data base searches based on homology to sequences of known ⌬ 5 -desaturase cDNAs from fungal and algal species. The resulting P. patens cDNA encodes a 480-amino acid polypeptide that contains a predicted N-terminal cytochrome b 5 -like domain as well as three histidine-rich domains. Expression of the enzyme in Saccharomyces cerevisiae resulted in the production of the ⌬ 5 -containing fatty acid arachidonic acid in cells that were provided di-homo-␥-linolenic acid. In addition, the expressed enzyme generated ⌬ 5 -desaturation products with the C20 substrates -6 eicosadienoic and -3 eicosatrienoic acids, but no products were detected with the C18 fatty acid linoleic and ␣-linolenic acids or with the C22 fatty acid adrenic and docosapentaenoic acids. When the corresponding P. patens genomic sequence was disrupted by replacement through homologous recombination, a dramatic alteration in the fatty acid composition was observed, i.e. an increase in di-homo-␥-linolenic and eicosatetraenoic acids accompanied by a concomitant disappearance of the ⌬ 5 -fatty acid arachidonic and eicosapentaenoic acids. In addition, overexpression of the P. patens cDNA in protoplasts isolated from a disrupted line resulted in the restoration of arachidonic acid synthesis.
Polyunsaturated fatty acids (PUFAs),4 fatty acids containing 18 or more carbon atoms with two or more methylene-interrupted double bonds in the cis-position, have received increasing attention in recent years because they are considered to have beneficial effects on human health and development when included in the diet (1, 2). Long-chain PUFAs (ՆC20) are not commonly found in angiosperms; however, they are present in some gymnosperms (3). High proportions of PUFA accumulation are found in many algae, mosses, and ferns (4 -6). The function of these long-chain PUFAs in the membranes of lower plants is still unclear, whereas in humans they play a role in eicosanoid metabolism (7). ) and EPA (20:5 ⌬ 5,8,11,14,17 ) are precursors of the short-lived regulatory molecules, the eicosanoids, that comprise the prostaglandins, the leukotrienes, and the thromboxanes (9 -11). These compounds act locally through autocrine or paracrine process on G-protein-linked cell surface receptors. This leads to the activation of various signaling mechanisms that have effects on numerous cellular functions including chemotaxis, vascular permeability, inflammation, vasoconstriction, regulation of the immune system, blood clotting, neurotransmission, and cholesterol metabolism (12)(13)(14). ARA (20:4 ⌬ 5,8,11,14 ) and EPA (20:5 ⌬ 5,8,11,14,17 ) are converted from linoleic acid (18:2 ⌬ 9,12) and ␣-linolenic acid (18:3 ⌬ 9,12,15 ), respectively, by the activities of ⌬
6-desatura...