Cysteine biosynthesis in Saccharomyces cerevisiae: A new outlook on pathway and regulation

Using a Saccharomyces cerevisiae strain having the activities of serine O-acetyl-transferase (SATase), O-acetylserine/O-acetylhomoserine sulphydrylase (OAS/OAH SHLase), cystathionine β-synthase (β-CTSase) and cystathionine γ-lyase (γ-CTLase), we individually disrupted CYS3 (coding for γ-CTLase) and CYS4 (coding for β-CTSase). The obtained gene disruptants were cysteine-dependent and incorporated the radioactivity of ³⁵S-sulphate into homocysteine but not into cysteine or glutathione. We concluded, therefore, that SATase and OAS/OAH SHLase do not constitute a cysteine biosynthetic pathway and that cysteine is synthesized exclusively through the pathway constituted with β-CTSase and γ-CTLase; note that OAS/OAH SHLase supplies homocysteine to this pathway by acting as OAH SHLase. From further investigation upon the cys3-disruptant, we obtained results consistent with our earlier suggestion that cysteine and OAS play central roles in the regulation of sulphate assimilation. In addition, we found that sulphate transport activity was not induced at all in the cys4-disruptant, suggesting that CYS4 plays a role in the regulation of sulphate assimilation.