Genetic variation in the cysteine biosynthesis pathway causes sensitivity to pharmacological compounds

Complex traits are the product of multiple genes with effects that depend on both the genetic and environmental background. Although this complexity makes a comprehensive genetic analysis difficult, identification of even a single gene provides insight into the biochemical and/or signaling pathway underlying a trait. However, it is unknown whether multiple pathways, and consequently multiple genes, must be identified to adequately understand a trait's molecular basis. Using crosses between three natural isolates of Saccharomyces cerevisiae, we mapped sensitivity to a number of pharmacologically active compounds to a single non-synonymous polymorphism in cystathione-beta-synthase (CYS4), which is required for the first committed step in the cysteine biosynthesis pathway. Drug sensitivity is mediated by a deficiency in cysteine and consequently glutathione production, because drug sensitivity is abrogated by cysteine or glutathione supplementation. Within a diverse panel of 60 natural yeast isolates, the drug-sensitive CYS4 allele is rare, and glutathione supplementation failed to alleviate drug-dependent growth defects in two other drug-sensitive strains. These results implicate the cysteine/glutathione biosynthesis pathway as a significant, but not the sole contributor to pharmacological variation in yeast.