SUMMARY: grown anaerobically in the absence of unsaturated fatty acids and sterol depleted its endogenous levels of these lipids by dilution as yeast numbers and mass increased. When the levels of unsaturated fatty acids and sterol reached approximately one-quarter those found in aerobically grown cells, the organisms stopped dividing; cells then ceased generating dry weight. Concomitant with this depletion of lipids was a decline in the protein-synthesizing activity of the cell. In mitochondria this was due to a loss of high molecular weight RNA. In the cytoplasm the effect was at the level of the ribosomes but was not due to loss of integrity of ribosomal or associated RNA, nor to a decrease in the ribosome/polysome complement. When oxygen was supplied, protein synthesis in mitochondria and cytoplasm was rapidly reactivated in the absence of cell growth. This reactivation was accompanied by a rapid resynthesis of unsaturated fatty acids and ergosterol.


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