A calcium-dependent ergosterol mutant of Saccharomyces cerevisiae.

ERG24 is the structural gene for the C14-sterol reductase in yeast. A lack of activity in that enzyme, mediated either by the morpholine fungicides or the insertional inactivation of ERG24, causes the accumulation of the aberrant sterol ignosterol. Cells producing this sterol are unable to grow aerobically in the routine laboratory medium, YPD. However, growth does occur on a synthetic defined medium. A novel calcium-dependent phenotype associated with alterations in the ergosterol biosynthetic pathway in yeast is described. In addition, reduction of yeast growth with an azole inhibitor of the C-14 sterol de-methylase was also modulated by an excess of calcium ions in the culture medium. These results define a new effect of ergosterol deficiency and provide important practical implications for utilizing morpholine and azole sterol biosynthetic-inhibiting fungicides.