Abstract1. The lipid composition of a mutant of Saccharomyces cerevisiae which cannot synthesize unsaturated fatty acid (UFA) can be extensively manipulated by growing the organism in the presence of added fatty acids.2. Growth of the mutant is supported by a wide range of unsaturated fatty acids including oleie, palmitoleic, petroselenic, l l-eicosaenoic, ricinoleic, araehidonic, clupanodonic, linoleie and linolenic acids; 9-and 10-hydroxystearic acids support growth less effectively, but erucic, nervonic, elaidic and saturated fatty acids (Cs:0-C20:0)* are ineffective. All the fatty acids which support growth are incorporated into cell lipids, apparently without further metabolism.3. The effects of altered lipid composition on the energy metabolism of yeast ceils were investigated. Cells containing less than approximately 20 % of their fatty acids as UFA cannot grow on non-fermentable substrates, and their growth on glucose is restricted to that which can be supported by fermentation alone.4. UFA-depleted cells contain mitochondria which are apparently normal in morphology, furthermore they have normal levels of cytochromes a + a3, b, cl and c and respire at normal rates. This suggests that the lesion in energy metabolism produced by UFAdepletion may be the loss of the ability of the mitochondria to couple respiration to phosphorylation.5. UFA-depleted cells incorporate added UFA into their cell lipids and subsequently regain the ability to grow on non-fermentable substrates, showing that the lesion in energy metabolism is fully reversible.

Abstract1. The lipid composition of mitochondria isolated from a fatty acid desaturase mutant of Saccharomyces cerevisiae may be extensively manipulated by growing the organism on defined supplements of unsaturated fatty acid (UFA).2. The fatty acid composition of the mitochondrial lipids closely follows that of the whole ceils from which the mitochondria are isolated. UFA-depleted mitochondria contain normal levels of sterols, neutral lipids and total phospholipids, but have much lower levels of phosphatidyl inositides.3. UFA-depleted mitochondria possess a full complement of cytochromes, oxidase both NAD-linked and flavoprotein-linked substrates at normal rates, and have levels of succinate and malate dehydrogenases similar to those of UFA-supplemented mitochondria. However, UFA-depletion has a marked effect on the ability ofcytochrome c to reactivate the NADH oxidase activity of cytochrome c-depleted mitochondria.4. The efficiency of oxidative phosphorylation decreases progressively with the UFA content of the mitochondria, and oxidative phosphorylation is completely lost in mitochondria containing approximately 20% UFA. The incorporation of UFA into the lipids of UFA-depleted mitochondria in vivo resultsin a recoupling of oxidative phosphorylation. Recoupling is insensitive to both chloramphenicol and cycloheximide, indicating that all the proteins necessary for oxidative phosphorylation are present in UFA-depleted mitochondria, and that the less of oxidative phosphorylation is a purely lipid lesion.6. ATPase activity is apparently unaffected by UFA-depletion, but 32pi-ATP exchange activity is lost in mitochondria which have been extensively depleted in UFA.7. Valinomycin stimulates the respiration of UFA-supplemented mitochondria in media containing potassium, but has no effect on the respiration of UFA-depleted mitochondria, suggesting that active transport of potassium is lost as a result of UFA-depletion.

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