The Enzymic Synthesis of l-Cysteine in Escherichia coli and Salmonella typhimurium

Abstract

Abstract A two-step pathway from l-serine to l-cysteine is described in Escherichia coli and Salmonella typhimurium. Serine transacetylase, the first enzyme in this pathway, catalyzes the formation of O-acetyl-l-serine from l-serine and acetyl coenzyme A and is inhibited by l-cysteine. The enzyme has been purified approximately 1000-fold and has a spectrum similar to that of pyridoxal phosphate-containing enzymes. O-Acetylserine sulfhydrylase, the second enzyme in the pathway, catalyzes the formation of l-cysteine from O-acetyl-l-serine and sulfide. It is repressed in Salmonella grown on l-cysteine and derepressed when grown on l-djenkolic acid. Cys E mutants in S. typhimurium contain low or undetectable levels of serine transacetylase and variable levels of O-acetylserine sulfhydrylase.