Nutritional regulation of yeast delta-9 fatty acid desaturase activity.

Abstract

The addition of unsaturated fatty acids to cultures of Saccharomyces cerevisiae significantly altered the microsomal lipid composition. Supplementation with either of the naturally occurring palmitoleic (16:1) or oleic (18:1) acids caused increased levels in membrane phospholipids and reduced levels of the complementary acid. Growth in the presence of equimolar quantities of 16:1 and 18:1 acids, however, produced a fatty acid composition similar to that found in unsupplemented cell membranes. Linoleic acid (18:2) was not found in S. cerevisiae grown under normal conditions. It was preferentially internalized and incorporated into microsomes, however, at levels exceeding 50% of the total fatty acid species. This resulted in an almost total loss of 16:1 and a reduction of 18:1 to 25% of its normal level. The delta-9 fatty acid desaturase, a microsomal enzyme that forms 16:1 and 18:1 from saturated acyl coenzyme A precursors, was affected by the presence of exogenous fatty acids. Enzyme activity toward the 16:0 coenzyme A substrate was elevated in microsomes from saturated-fatty-acid-supplemented cultures and sharply repressed following the addition of unsaturated fatty acids, including 18:2. Northern (RNA blot) and slot-blot analyses of mRNA encoded by the OLE1 gene, which appears to be the structural gene for the delta-9 desaturase, indicated that it was sharply reduced in unsaturated-fatty-acid-fed cells. These data suggest that a significant part of the regulation involves modulation of available transcripts.