Transcriptional regulation of the two sterol esterification genes in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae transcribes two genes, ARE1 and ARE2, that contrib ute disproportionately to the esterification of sterols. Are2p is the major enzyme isoform in a wild-type cell growing aerobically. This likely result s from a combination of differential transcription initiation and transcrip t stability. By using ARE1 and ARE2 promoter fusions to lacZ reporters, we demonstrated that transcriptional initiation from the ARE] promoter is sign ificantly reduced compared to that from the ARE2 promoter. Furthermore, the half-life of the ARE2 mRNA is approximately 12 times as long as that of th e ARE1 transcript. We present evidence that the primary role of the minor s terol esterification isoform encoded by ARE] is to esterify sterol intermed iates, whereas the role of the ARE2 enzyme is to esterify ergosterol, the e nd product of the pathway. Accordingly, the ARE] promoter is upregulated in strains that accumulate ergosterol precursors. Furthermore, ARE1 and ARE2 are oppositely regulated by heme. Under heme-deficient growth conditions, A RE1 was upregulated fivefold while ARE2 was down-regulated. ARE2 requires t he HAP1 transcription factor for optimal expression, and both ARE genes are derepressed in a rox1 (repressor of oxygen) mutant genetic background. We further report that the ARE genes are not subject to end product inhibition ; neither ARE] nor ARE2 transcription is altered in an are mutant backgroun d, nor does overexpression of eitherARE gene alter the response of the ARE- lacZ reporter constructs. Our observations are consistent with an important physiological role for Are1p during anaerobic growth when heme is limiting and sterol precursors may accumulate. Conversely, Are2p is optimally requi red during aerobiosis when ergosterol is plentiful.