New Insight into Phaeodactylum Tricornutum Fatty Acid Metabolism. Cloning and Functional Characterization of Plastidial and Microsomal δ12-Fatty Acid Desaturases

• Published in: Plant physiology (Bethesda) 2003-04, Vol.131 (4), p.1648-1660
• Main Authors: Domergue, Frédéric, Patricia Spiekermann, Lerchl, Jens, Christoph Beckmann, Oliver Kilian, Peter G. Kroth, Wilhem Boland, Zähringer, Ulrich, Heinz, Ernst
• Format: Article
• Language: English
• Subjects: Agronomy. Soil science and plant productions; Biochemical Processes and Macromolecular Structures; Biological and medical sciences; Canola; Chemical desaturation; Diatoms; Economic plant physiology; Enzymes; Fatty acids; Fundamental and applied biological sciences. Psychology; Metabolism; Metabolism. Physicochemical requirements; Nitrogen metabolism and other ones (excepting carbon metabolism); Nutrition. Photosynthesis. Respiration. Metabolism; Open reading frames; Plant physiology and development; Plastids; Proteins; Substrate specificity; Yeasts
• ISSN: 0032-0889; 1532-2548
• DOI: 10.1104/pp.102.018317

In contrast to 16:3 plants like rapeseed (Brassica napus), which contain α-linolenic acid ($18\colon 3^{\Delta 9,12,15}$) and hexadecatrienoic acid ($16\colon 3^{\Delta 7,10,13}$) as major polyunsaturated fatty acids in leaves, the silica-less diatom Phaeodactylum tricornutum contains eicosapentaenoic acid (EPA; $20\colon 5^{\Delta 5,8,11,14,17}$) and a different isomer of hexadecatrienoic acid ($16\colon 3^{\Delta 6,9,12}$). In this report, we describe the characterization of two cDNAs having sequence homology to Δ12-fatty acid desaturases from higher plants. These cDNAs were shown to code for a microsomal and a plastidial Δ12-desaturase (PtFAD2 and PtFAD6, respectively) by heterologous expression in yeast (Saccharomyces cerevisiae) and Synechococcus, respectively. Using these systems in the presence of exogenously supplied fatty acids, the substrate specificities of the two desaturases were determined and compared with those of the corresponding rapeseed enzymes (BnFAD2 and BnFAD6). The microsomal desaturases were similarly specific for oleic acid ($18\colon 1^{\Delta 9}$), suggesting that PtFAD2 is involved in the biosynthesis of EPA. In contrast, the plastidial desaturase from the higher plant and the diatom clearly differed. Although the rapeseed plastidial desaturase showed high activity toward the ω9-fatty acids $18\colon 1^{\Delta 9}$ and $16\colon 1^{\Delta 7}$, in line with the fatty acid composition of rapeseed leaves, the enzyme of P. tricornutum was highly specific for $16\colon 1^{\Delta 9}$. Our results indicate that in contrast to EPA, which is synthesized in the microsomes, the hexadecatrienoic acid isomer found in P. tricornutum ($16\colon 3^{\Delta 6,9,12}$) is of plastidial origin.