Graduate Thesis Or Dissertation
 

Physiological, biochemical, and genetic studies on the [delta]²⁴ sterol methyltransferase of Saccharomyces cerevisiae

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  • The role of the yeast S-adenosylmethionine: Δ²⁴-sterol-C-methyltransferase (SCMT) in sterol metabolism was investigated. Structural analogs of S-adenosylhomocysteine were tested for inhibition of the SCMT enzyme. A wide inhibitory range by these compounds was observed, indicating which structural features of the parent compound are important for binding to the enzyme. The most active compound, sinefungin, was able to inhibit growth of yeast cultures and cause the accumulation of the SCMT sterol substrate, zymosterol. Sinefungin is transported inside the cell by the S-adenosylmethionine permease and affects the metabolism of this compound intracellularly. The genetic organization of mutants defective in sterol methylation was examined. Mutations isolated from different parentals were demonstrated to be from the same gene, erg6. This mutation was mapped to chromosome four of the yeast genome, 18.0 cM from the trpl locus. The lipids of mutants defective in the SCMT or in 5(6) desaturation (erg3) were analyzed. The distribution of ergosterol precursors accumulated by these mutants varied with growth phase and between free sterols and steryl esters. The steryl ester concentrations of the mutants were eight times higher than the wild type from exponential growth samples. When the culture entered stationary phase, the ester concentration increased seven fold in the parental, but less than two fold in the mutants. These results suggest that the regulation of ester synthesis is defective in the mutants. The sterol mutants also had elevated lipid synthetic rates, implicating a role for ergosterol in modulating cellular lipid synthesis. While the head group phospholipid composition was the same between parentals and mutants, strain-dependent changes in fatty acids were observed, most notably a 40% increase in the oleic acid content of phosphatidylethanolamine of one erg6 mutant, JR5. Whole cell incorporation of methionine in sterols of JR5 was less than 0.2% that of the parental, X2180-1B. No ergosterol (C- 28) could be detected in whole cell sterol extracts of JR5, the limits of detection being less than 10⁻¹¹ moles of ergosterol per 10⁸ cells. It can be concluded that little if any C-28 sterol is required for growth of S. cerevisiae.
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