External factors regulate the expression of genes for fatty acid desaturases

Changes in the level of unsaturation, due to addition or depletion of double bonds in the backbone of membrane fatty acids (FAs), have been demonstrated to be important deterrninants of membrane stability and functionality under changing extemaI conditions in lower and higher organisms, included plants. The FA unsaturation is cataIyzed by desaturases, which introduce double bonds between two carbons ofFA acyl chains. Two cDNAs, encoding a chIoroplastic delta9 and a ER-bound delta12 desaturase respectively, have been cloned from a potato library and characterized. The ORF of clone pScDes4.1, encoding a delta9 -stearoyl- desaturase is 393 amino acid long, including 30 amino acid residues corresponding to a putative chIoroplastic transit peptide. The ORF ofpScDll, encoding a delta12 -oleoyl-desaturase is 383 amino acid residue long. Sequence anaIysis of this clone identified histidine-rich motifs, highIy conserved in other desaturases, and six-putative membrane spannings. The level of expression of the two genes was found to be down-regulated in potato cells adapted to PEG- induced water deficit as well as in response to a graduaI temperature up-shift. The reduced level of expression correlated with a higher degree of membrane saturation as well as with an expected iricrease in membrane microviscosity. However neither abrupt osmotic shock nor heat shock triggered changes in the expression of these two genes, indicating that biophysical modifications of membrane only occur as a long-term response to prolonged and graduaI exposure to stress.