AbstractRegulation of the two enzymes in reverse trans‐sulfuration was investigated in Saccharomyces cerevisiae. In wild‐type strains, cystathionine γ‐lyase, but not cystathionine β‐synthase, was derepressed nearly 15‐fold if cells were starved for both inorganic and organic sulfur compounds. In a met17 strain which is defective of O‐acetylserine and O‐acetylhomoserine sulfhydrylase, the same enzyme was derepressed if organic sulfur compounds were limited; the repressive effect was in the order of glutathione > methionine > cysteine. The repressive effect of methionine was not observed, however, in a cys2 cys4 strain which is deficient of serine O‐acetyltransferase and cystathionine β‐synthase, indicating that methionine itself is not the effector. The weak repressive effect of cysteine was attributed to inefficient uptake of this amino acid.Our observations indicate that cystathionine γ‐lyase is the target of regulation in reverse trans‐sulfuration and that cysteine is very likely to be the effector of this regulation.