Structural discrimination in the sparking function of sterols in the yeast Saccharomyces cerevisiae

A Saccharomyces cerevisiae sterol auxotroph, SPK14 (a hem1 erg6 erg7 ura), was constructed to test the ability of selected C-5,6 unsaturated sterols at growth-limiting concentrations to spark growth on bulk cholestanol. The native sterol, ergosterol, initiated growth faster and allowed a greater cell yield than did other sterols selectively altered in one or more features of the sterol. Although the C-5,6 unsaturation is required for the sparking function, the presence of the C-22 unsaturation was found to facilitate sparking far better than did the C-7 unsaturation, whereas the C-24 methyl was the least important group. The addition of delta-aminolevulinic acid to the medium allowed the sparking of FY3 (hem1 erg7 ura) on bulk cholestanol due to the derepression of 3-hydroxy-3-methylglutaryl-coenzyme A reductase and the production of endogenous ergosterol. The optimal concentration of delta-aminolevulinic acid to spark growth was 800 ng/ml, whereas higher concentrations caused a growth inhibition. The growth yield of FY3 reached a plateau maximum at about 5 micrograms/ml when the bulk cholestanol was varied in the presence of 10 ng of sparking erogosterol per ml.