Molecular cloning and functional expression of the gene for a cotton Δ-12 fatty acid desaturase (FAD2)

Two overlapping genomic clones spanning 16.5 kb of cotton DNA were found to encompass a Δ-12 fatty acid desaturase (FAD2-3) gene. A partial FAD2-3 cDNA clone was also analyzed. The FAD2-3 gene has one large intron of 2967 bp entirely within its 5′-untranslated region, only 12 bp upstream from the ATG initiation codon. Several potential promoter elements, including several light-responsive motifs, occur in the 5′-flanking region. The continuous FAD2-3 coding region is 1155 bp and would encode a protein of 384 amino acids. The polypeptide has four putative membrane-spanning helices, indicative of an integral membrane protein, and is most likely localized in the endoplasmic reticulum. Yeast cells transformed with a plasmid construct containing the cotton FAD2-3 coding region accumulate an appreciable amount of linoleic acid (18:2), not normally present in wild-type yeast cells, indicating that the gene encodes a functional FAD2 enzyme.