PHYTOCHELATIN-MEDIATED CADMIUM TOLERANCE IN SCHIZOSACCHAROMYCES-POMBE

Plants and certain fungi respond to heavy metal toxicity with the indu ced synthesis of metal-binding peptides known as phytochelatins (PCs). With cadmium, PCs can bind the metal to form a low molecular weight P C-Cd complex and a high molecular weight PC-Cd-S2- complex. The sulfid e ions enhance the stability and Cd-binding capacity of the metal chel ate, and formation of this sulfide-containing complex is associated wi th enhanced tolerance to cadmium. Molecular analyses of two fission ye ast mutants that fail to produce a wild type level of the PC-Cd-S2- co mplex have determined that a vacuolar membrane transporter and several enzymes of the purine biosynthesis pathway are necessary in vivo for formation of the PC-Cd-S2- complex. A model based on vacuolar sequestr ation of the PC-Cd complex by an ATP-binding cassette-type transporter and its subsequent maturation into the stable PC-Cd-S2- complex via t he actions of two purine biosynthetic enzymes is described.