Genes for fatty acid desaturase: control of the expression by external factors

The conservation of a particular state of membrane fluidity upon environmental changes, which is critical for a wide spectrum of membrane-associated protein functions, may be accomplished by regulating the degree of unstaturation of the membrane fatty acids (FAs). The FA unsaturation in plants is controlled by a set of desaturases, which catalyze the introduction of a double bond between two carbons of the acyl chain. The first step of C18 FA unsaturation takes place in the stroma by the stearoyl-ACP desaturase, which introduces a double bond at 9 position of stearoyl-ACP to produce oleoyl-ACP. Further unsaturation is catalyzed by other cytoplasmatic membrane-bound desaturase only after the transfer of oleic acid (18:1) to a glycerol moiety, leading to linoleic acid (18:2) and linolenic (18:3) by subsequent steps of unsaturation catalyzed by a 12 and a 15 desaturase respectively. We have cloned recently two full-length cDNAs encoding a chloroplastic 9 and a ER-bound 12 desaturase respectively, from a library of a wild potato species. The level of expression of these two genes was found to be down-regulated during acclimation of potato cells to PEG-induced water stress or after an upward temperature shift. In PEG acclimated cells, the down-regulation of these two genees corresponded to a lower degree of membrand FA unsaturation as well as to an expected increase in membrane microviscosity. Changes in the expression of these two genes were triggered neither by abrupt osmotic stress nor by heat shock, indicating that biophysical membrane modifications are time-dependent and only occur after prolonged and gradual exposure to the stress.