in metals (e.g., mine soils and deposits) or even directly on metal veins [41,73]. This adaptation of plants
is not well understood. It was shown, however, that they are particularly enriched in PCs. These polypep-
tides are found in roots and stems of higher plants but not in leaves or fruits [74].
PCs are characterized by the general structure ( -Glu-Cys)n-Gly, where n2–11 (Figure 1). They
can, however, differ in their C-terminal amino acid [3,54]. The cystein residues ensure metal coordina-
tion via their thiol group. What were called cadystins for Cd-containing complexes in the fission yeast
Schizosaccharomyces pombe[75] correspond in fact to PC 2 and PC 3. The high percentage of glutamic
acid residues makes PCs extremely water soluble, explaining why they are usually localized in the cyto-
plasm. Optimized structures of different Cd-PC complexes have been proposed [76].
The fact that amino acids of PCs are linked by the -carbon of the carboxylic acid residue of gluta-
mate excludes the usual protein synthesis via translation in ribosomes [77]. Actually, the PC production
starts with two molecules of glutathione -Glu-Cys-Gly with the loss of one Gly to form the smallest PC
(n2) [78,79]. To increase n, more free glutathione (a major intracellular reductant) is required; every
HOW PLANTS ADAPT TO EXCESS OF METALS 755
Figure 1 (A) Comparison of -Glu-Cys-Gly and -Glu-Cys-Gly (glutathione). (B) Structure of PC com-
posed of three ( -glutamylcysteinyl) units and cadystin A.