BIOSYNTHESIS AND METABOLIC ROLES OF CADYSTINS (GAMMA-EC)NG AND THEIR PRECURSORS IN DATURA-INNOXIA

Metal-tolerant Datura innoxia cells synthesize large amounts of cadystin, [poly(gamma-glutamylcysteinyl) glycines, (gamma-EC)(n)G, n = 2 - 5], a class of metal-binding polypeptides, when exposed to Cd. These polypeptides have a high affinity for Cd (II) and certain other metal ions and are thought to play a role in metal tolerance in higher plants. Cells rapidly synthesize these metal-binding polypeptides when exposed to Cd and cellular concentrations of glutathione and cysteine, precursors for the synthesis of these compounds, are initially depleted then rapidly replenished. The time-frame of de novo polypeptide, glutathione and cysteine biosynthesis suggests that this pathway is, at least initially, regulated at the enzyme level. Significant amounts of Fe are associated with Cd: polypeptide complexes isolated from D. innoxia. Exposure of cultures to Cd results in an increased Fe accumulation by the cells. All the additional Fe found in the soluble portion of cell extracts is associated with the Cd: polypeptide complexes. The physiological significance of the synthesis of these polypeptides and their precursors and its relevance to Cd tolerance and metal homeostasis are discussed.