Phytochelatin synthase genes from arabidopsis and the yeast Schizosaccharomyces pombe

Phytochelatins (PCs), a family of heavy metal-inducible peptides important in the detoxification of heavy metals, have been identified in plants and s ome microorganisms, including Schizosaccharomyces pombe, but not in animals . PCs are synthesized enzymatically from glutathione (GSH) by PC synthase i n the presence of heavy metal ions. In Arabidopsis, the CAD1 gene, identifi ed by using Cd-sensitive, PC-deficient cad1 mutants, has been proposed to e ncode PC synthase. Using a positional cloning strategy, we have isolated th e CAD1 gene. Database searches identified a homologous gene in S. pombe, an d a mutant with a targeted deletion of this gene was also Cd sensitive and PC deficient, Extracts of Escherichia coli cells expressing a CAD1 cDNA or the S. pombe gene catalyzing GSH-dependent, heavy metal-activated synthesis of PCs in vitro demonstrated that both genes encode PC synthase activity. Both enzymes were activated by a range of metal ions. In contrast, reverse transcription-polymerase chain reaction experiments showed that expression of the CAD1 mRNA is not influenced by the presence of Cd, A comparison of t he two predicted amino acid sequences revealed a highly conserved N-termina l region, which is presumed to be the catalytic domain, and a variable C-te rminal region containing multiple Cys residues, which is proposed to be inv olved in activation of the enzyme by metal ions. Interestingly, a similar g ene was identified in the nematode, Caenorhabditis elegans, suggesting that PCs may also be expressed in some animal species.