O-acetylserine (thiol) lyase (EC 4.2.99.8) in green and non green tissues from higher plants has been located predominantly in the plastids, the cytosol, but is also present in the mitochondria. The chloroplastic isoform of O-acetylserine (thiol) lyase from plant tissue has been purified to homogeneity. We have isolated and characterized a cDNA from a ?igt 11 spinach library encoding the complete chloroplastic O-acetylserine (thiol) lyase. The deduced primary sequence revealed that the 331 amino acid polypeptide is cytoplasmically synthesized with a 52 amino acid targetting peptide. We also report the presence, in chloroplasts, of a multifunctional protein complex which links serine to cysteine synthesis as previously characterized in bacteria.