Role of Membrane Phospholipid Biosynthesis in Cellular Responce to Enviornmental Stress and Start of Cell Division Cycle in Yeast.

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(1) Analysis of role of nitrogen-containing phospholipids in yeast cell growth.(1) Response to various environmental conditions of a yeast mutant that were deficient in phosphatidylserine (PS) synthesis were examined. Even in the presence of choline or ethanolamine which enables the mutant to grow, only 10% cells were viable at mid-stationary phase, judging by Malachite green staining. Sensitivity to high concentration of Ca^<2+>, Zn^<2+>, Mn^<2+>, or basic amino acids was increased. Central vacuoles were not observed by Nomarski optics and fluorescence microscopy of quinacrine-stained cells. These results suggest that PS aynthesis is necessary for the function of intracellular membraneous structures and for cellular adaptation to environmental conditions.(2) A strain of which chromosomal CHO1 gene that encoded PS synthase was disrupted but that carried it downstream of GAL7 promoter on a centromere plasmid was constructed. When the strain was transferred from a medium with galactose to the one with glucose as a carbon source, the amount of PS was decreased to 0.3% of total phospholipids and that of phosphatidylethanolamine was also decreased, but that of posphatidylinositol was increased up to 47% at 9 hours after C-source change. Increase in culture turbidity was continued for 9 hours after medium change, but the increase in cell number did not continue over 6 hours, when 80% to 90% of cells were unbudded cells. This result suggests that the cessation of PS synthesis blocked entry from G1 to S phase of cell division cycle.(2) Analysis of upstream region of the CHO1 gene(1) Expression of a CHO1-lacZ fusion gene was examined after heat shock. Since stimulation of reporter enzyme activity was not observed, role of an HSE-like sequence in 5'-upstream region was not clear.