PLEIOTROPIC EFFECTS OF THE OPI1 REGULATORY MUTATION OF YEAST - ITS EFFECTS ON GROWTH AND ON PHOSPHOLIPID AND INOSITOL METABOLISM

Key factors which impact on the biosynthesis and subsequent fate of th e phospholipid precursor inositol were studied as a function of growth phase in the yeast Saccharomyces cerevisiae, Both wild-type and strai ns disrupted for the OPI1 gene, the principal negative regulator of th e phospholipid biosynthetic genes, were examined. Overexpression of th e INO1 gene and overproduction of both inositol and the major inositol -containing phospholipid, phosphatidylinositol. varied as a function o f growth phase. In opi1 cells, INO1 expression was constitutive at a h igh level throughout growth, although the level of transcript was redu ced at stationary phase when the cells were grown in defined medium. I n the wild-type strain, INO1 expression was limited to a peak in the e xponential phase of growth in cells grown in the absence of inositol, Interestingly, the pattern of OPI1 expression in the wildtype strain r esembled that of its putative target, INO1. Intracellular inositol con tents of the opi1 strain were higher than those of the wild-type strai n, with peak levels occurring in the stationary phase. Membrane phosph atidylinositol content paralleled intracellular inositol content, with opi1 strains having a higher phosphatidylinositol content in stationa ry phase. The proportion of the predominant phospholipid, phosphatidyl choline, exhibited a profile that was the inverse of the phosphatidyli nositol content: phosphatidylcholine content was lowest in opi1 cells in stationary phase. The opi1 mutation was also found to have effects beyond phospholipid biosynthesis, opi1 cells were smaller, and opi1 cu ltures achieved a cell density twice as high as comparable wild-type c ultures, opi1 cells were also more salt tolerant than wild-type cells: they were partly resistant to shrinking, more rapidly resumed growth, and attained a higher culture density after upshift to medium supplem ented with 8% NaCl.