Glutathione (L-y-glutamyl-L-cysteinylglycine, GSH) plays an important role in the protection of plants against various types of stress caused by reactive oxygen species, gazeous pollutants, heavy metals and xenobiotics. A cDNA fragment containing the entire coding unit for glutathione synthetase (GSH2) of Ambidopsis thaliunu was cloned by complementation of the methylglyoxal sensitivity of a gsh2 mutant of the yeast Succharomyces cerevisiae. The cDNA encodes a protein of 478 amino acids (deduced M,: 53 783), bearing clear sequence similarities to GSH2 products from frog embryos (Xenopus laevis), rat kidney (Rattus norvegicus) and from the fission yeast (Schizosaccharonzyces pombe). A highly conserved glycine-rich domain close to the carboxy-terminus was found in the GSH2 product and appears to be typical for eukaryotic glutathione synthetases. The M , is similar to those of soluble animal enzymes, suggesting that the Amhidopsis gene also codes for a cytosolic protein. Genomic DNA-blot analysis indicates the presence of a single GSH2 gene. The yeast gsh2 mutant becomes resistant to methylglyoxal and cadmium after transformation with the plasmid bearing the Arabidopsis GSH2 cDNA. Moreover, this increased resistance is correlated to the restoration of GSH content from below detectibility in mutants to about 50% of the wild-type levels in transformed cells.Keywords: glutathione synthetase; expression cloning; Arabidopsis thaliana; evolutionary relationships; cadmium.Glutathione (L-y-glutamyl-L-cysteinylglycine, GSH) is an ubiquitous tripeptide (Rennenberg, 1982;Fahey and Sundquist, 1991: Penninckx andElskens, 1993) that plays an important role in the protection of plants against various types of stress caused by reactive oxygen species and gazeous pollutants (Alscher, 19891, low temperature (De Kok and Oosterhuis, 1983) and heavy metals (Grill et al., 1985). GSH is also involved in the detoxification of xenobiotics through formation of conjugates via the action of' various GSH S-transferases (Timmerman, 1989). For some safeners, compounds which increase the selectivity of some herbicides, it was reported that the protection of the crop plant is partially due to a stimulation of GSH accumulation (Farago et al., 1994), followed by increased conjugation and sequestration into the vacuole (Gaillard et al., 1994). Furthermore, GSH, which is chemically stable, represents the most abundant form of reduced sulphur within the cell and, in plants, it appears to be involved in the control of uptake and xylem loading of sulfate (Herschbach and Rennenberg, 1994 Ahhreviations. GSH, glutathione; GSH-I, y-glutamylcysteine synthetase; GSHI, y-glutamylcysteine synthetase gene; GSH-11, glutathione synthetase; GSH2, glutathione synthetase gene.Enzymes. Glutathione synthetase, y-L-glutamyl-L-cysteine: glycine ligase (ADP forming) (EC 6.3.2.3); pglutamylcysteine synthetase (EC 6.3.2.2).Note. The nucleotide sequence mentioned here has been submitted to the EMBL sequence data bank and is available under accession number XX3411.GSH is syn...