ARABIDOPSIS FAD2 GENE ENCODES THE ENZYME THAT IS ESSENTIAL FOR POLYUNSATURATED LIPID-SYNTHESIS

The polyunsaturated fatty acids linoleate and a-linolenate are importa nt membrane components and are the essential fatty acids of human nutr ition. The major enzyme responsible for the synthesis of these compoun ds is the plant oleate desaturase of the endoplasmic reticulum, and it s activity is controlled in Arabidopsis by the fatty acid desaturation 2 (fad2) locus. A fad2 allele was identified in a population of Arabi dopsis in which mutations had been created by T-DNA insertions. Genomi c DNA flanking the T-DNA was cloned by plasmid rescue and used to isol ate cDNA and genomic clones of FAD2. A cDNA containing the entire FAD2 coding sequence was expressed in fad2 mutant plants and shown to comp lement the mutant fatty acid phenotype. The deduced amino acid sequenc e from the cDNA showed homology to other plant desaturases, and this c onfirmed that FAD2 is the structural gene for the desaturase. Gel blot analyses of FAD2 mRNA levels showed that the gene is expressed throug hout the plant and suggest that transcript levels are in excess of the amount needed to account for oleate desaturation. Sequence analysis i dentified histidine-rich motifs that could contribute to an iron bindi ng site in the cytoplasmic domain of the protein. Such a position woul d facilitate interaction between the desaturase and cytochrome b(5), w hich is the direct source of electrons for the desaturation reaction, but would limit interaction of the active site with the fatty acyl sub strate.