Certain exogenously-supplied sterols, like ergost-8-enol, are efficiently converted into ergosterol in yeast. We have taken advantage of this property to study the regulation of the Δ8-Δ7-sterol isomerase-encoding ERG2 gene in an ergosterol auxotrophic mutant devoid of squalene-synthase activity. Ergosterol starvation leads to an 8–16-fold increase in ERG2 gene expression. Such an increase was also observed in wild-type cells either grown anaerobically or treated with SR31747A a sterol isomerase inhibitor. Exogenously-supplied zymosterol is entirely transformed into ergosterol, which represses ERG2 transcription. By contrast, exogenously-supplied ergosterol has little or no effect on ERG2 transcription.
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