Résumé
Phytochelatins, a class of posttranslationally synthesized peptides, play a pivotal role in heavy metal, primarily Cd2+, tolerance in plants and fungi by chelating these substances and decreasing their free concentrations. Derived from glutathione and related thiols by the action of γ-glutamylcysteine dipeptidyl transpeptidases (phytochelatin synthases; EC 2.3.2.15), phytochelatins consist of repeating units of γ-glutamylcysteine followed by a C-terminal Gly, Ser, or β-Ala residue [poly-(γ-Glu-Cys)n-Xaa]. Here we report the suppression cloning of a cDNA (AtPCS1) from Arabidopsis thaliana encoding a 55-kDa soluble protein that enhances heavy-metal tolerance and elicits Cd2+-activated phytochelatin accumulation when expressed in Saccharomyces cerevisiae. On the basis of these properties and the sufficiency of immunoaffinity-purified epitope-tagged AtPCS1 polypeptide for high rates of Cd2+-activated phytochelatin synthesis from glutathione in vitro, AtPCS1 is concluded to encode the enzyme phytochelatin synthase.