The Primary Structure of a cDNA Clone of the Stearoyl-Acyl Carrier Protein Desaturase Gene from Potato (Solanum tuberosum L.).

In plants, fatty acid biosynthesis takes place in the plastid. The primary products of fatty acid biosynthesis are palmitoylACP2 (16:0-ACP) and stearoyl-ACP (18:0-ACP). The soluble plastidic enzyme stearoyl-ACP desaturase (EC 1.14.99.6) introduces the first cis-double bond into stearoyl-ACP between carbons 9 and 10 to produce oleoyl-ACP (18:19-ACP). Further desaturation of palmitic or oleic acid occurs after the incorporation of the fatty acid into lipid. Most plants lack other desaturase activity that acts on stearic acid, and therefore, the overall level of fatty acid desaturation is strongly influenced by the activity of the stearoyl-ACP desaturase. The plant stearoyl-ACP desaturase has been purified from safflower and avocado. Both enzymes are soluble dimers of 70 kD and require reduced ferredoxin and molecular oxygen as cofactors. The use of polyclonal antibodies raised against these enzymes has enabled the isolation of cDNA clones from Brassica napus, castor (Ricinus communis), cucumber (Cucumis sativus), and safflower (Carthamus tinctorius) embryo-derived libraries. Shown in Figure 1 is the nucleotide and deduced amino acid sequence of a full-length cDNA of the potato stearoylACP desaturase gene. The clone was isolated accidentally while screening a potato stolon-tip library (constructed in lambda Zap II) with polyclonal antibodies raised against potato fructokinase. A putative ATG start codon is present in optimal context for translation initiation (underlined in Fig. 1). The sequence contains an open reading frame of 394 amino acids, similar to the size of other stearoyl-ACP desaturases (396-399 amino acids). There is considerable homology with stearoyl-ACP desaturases at the deduced amino acid sequence level, the potato gene showing 77% identity with the castor and 75% identity with the safflower sequences. In common with other desaturases, the region of greatest divergence occurs near the amino terminus in the putative transit peptide.