Cloning and characterization of the CYS3 (CYI1) gene of Saccharomyces cerevisiae

A DNA fragment containing the Saccharomyces cerevisiae CYS3 (CYI1) gene was cloned. The clone had a single open reading frame of 1,182 bp (394 amino acid residues). By comparison of the deduced amino acid sequence with the N- terminal amino acid sequence of cystathionine γ-lyase, CYS3 (CYI1) was concluded to be the structural gene for this enzyme. In addition, the deduced sequence showed homology with the following enzymes: rat cystathionine γ- lyase (41%), Escherichia coli cystathionine γ-synthase (36%), and cystathionine β-lyase (25%). The N-terminal half of it was homologous (39%) with the N-terminal half of S. cerevisiae O-acetylserine and O- acetylhomoserine sulfhydrylase. The cloned CYS3 (CYI1) gene marginally complemented the E. coli metB mutation (cystathionine γ-synthase deficiency) and conferred cystathionine γ-synthase activity as well as cystathionine γ- lyase activity to E. coli; cystathionine γ-synthase activity was detected when O-succinylhomoserine but not O-acetylhomoserine was used as substrate. We therefore conclude that S. cerevisiae cystathionine γ-lyase and E. coli cystathionine γ-synthase are homologous in both structure and in vitro function and propose that their different in vivo functions are due to the unavailability of O-succinyl-homoserine in S. cerevisiae and the scarceness of cystathionine in E. coli.