THE BIOGENESIS OF MITOCHONDRIA

Abstract

The growth conditions known to influence the occurrence of mitochondrial profiles and other cell membrane systems in anaerobic cells of S. cerevisiae have been examined, and the effect of the several growth media on the lipid composition of the organism has been determined. The anaerobic cell type containing neither detectable mitochondrial profiles nor the large cell vacuole may be obtained by the culture of the organism on growth-limiting levels of the lipids, ergosterol, and unsaturated fatty acids. Under these conditions, the organism has a high content of short-chain saturated fatty acids (10:0, 12:0), phosphatidyl choline, and squalene, compared with aerobically grown cells, and it is especially low in phosphatidyl ethanolamine and the glycerol phosphatides (phosphatidyl glycerol + cardiolipin). The high levels of unsaturated fatty acids normally found in the phospholipids of the aerobic cells are largely replaced by the short-chain saturated acids, even though the phospholipid fraction contains virtually all of the small amounts of unsaturated fatty acid present in the anaerobic cells. Such anaerobic cells may contain as little as 0.12 mg of ergosterol per g dry weight of cells while the aerobic cells contain about 6 mg of ergosterol per g dry weight. Anaerobic cell types containing mitochondrial profiles can be obtained by the culture of the organism in the presence of excess quantities of ergosterol and unsaturated fatty acids. Such cells have increased levels of total phospholipid, ergosterol, and unsaturated fatty acids, although these compounds do not reach the levels found in aerobic cells. The level of ergosterol in anaerobic cells is markedly influenced by the nature of the carbohydrate in the medium; those cells grown on galactose media supplemented with ergosterol and unsaturated fatty acids have well defined mitochondrial profiles and an ergosterol content (2 mg per g dry weight of cells) three times that of equivalent glucose-grown cells which have poorly defined organelle profiles. Anaerobic cells which are low in ergosterol synthesize increased amounts of squalene.