Coupling of the transcriptional regulation of glutathione biosynthesis to the availability of glutathione and methionine via the Met4 and Yap1 transcription factors.

Depletion of the cellular pool of glutathione is detrimental to eukaryotic cells and in Saccharomyces cerevisiae leads to sensitivity to oxidants and xenobiotics and an eventual cell cycle arrest. Here, we show that the Yap1 and Met4 transcription factors regulate the expression of gamma-glutamylcysteine synthetase (GSH1), encoding the rate-limiting enzyme in glutathione biosynthesis to prevent the damaging effects of glutathione depletion. Transcriptional profiling of a gsh1 mutant indicates that glutathione depletion leads to a general activation of Yap1 target genes, but the expression of Met4-regulated genes remains unaltered. Glutathione depletion appears to result in Yap1 activation via oxidation of thioredoxins, which normally act to down-regulate the Yap1-mediated response. The requirement for Met4 in regulating GSH1 expression is lost in the absence of the centromere-binding protein Cbf1. In contrast, the Yap1-mediated effect is unaffected, indicating that Met4 acts via Cbf1 to regulate the Yap1-mediated induction of GSH1 expression in response to glutathione depletion. Furthermore, yeast cells exposed to the xenobiotic 1-chloro-2,4-dintrobenzene are rapidly depleted of glutathione, accumulate oxidized thioredoxins, and elicit the Yap1/Met4-dependent transcriptional response of GSH1. The addition of methionine, which promotes Met4 ubiquitination and inactivation, specifically represses GSH1 expression after 1-chloro-2,4-dintrobenzene exposure but does not affect Yap1 activation. These results indicate that the Yap1-dependent activation of GSH1 expression in response to glutathione depletion is regulated by the sulfur status of the cell through a specific Met4-dependent mechanism.