Heavy Metal-Activated Synthesis of Peptides in Chlamydomonas reinhardtii

In this study, we have addressed the capacity of the green alga Chlamydomonas reinhardtii to produce metal-binding peptides in response to stress induced by the heavy metals Cd(2+), Hg(2+), and Ag(+). Cells cultured in the presence of sublethal concentrations of Cd(2+) synthesized and accumulated oligopeptides consisting solely of glutamic acid, cysteine, and glycine in an average ratio of 3:3:1. Cadmium-induced peptides were isolated in their native form as higher molecular weight peptide-metal complexes with an apparent molecular weight of approximately 6.5 x 10(3). The isolated complex bound cadmium (as evidenced by absorption spectroscopy) and sequestered (with a stoichiometry of 0.7 moles of cadmium per mole of cysteine) up to 70% of the total cadmium found in extracts of cadmium-treated cells. In Hg(2+)-treated cells, the principal thiol-containing compound induced by Hg(2+) ions was glutathione. It is possible that glutathione functions in plant cells (as it does in animal cells) to detoxify heavy metals. Cells treated with Ag(+) ions also synthesized a sulfur-containing component with a charge to mass ratio similar to Cd(2+)-induced peptides. But, in contrast to the results obtained using Cd(2+) as an inducer, these molecules did not accumulate to significant levels in Ag(+)-treated cells. The presence of physiological concentrations of Cu(2+) in the growth medium blocked the synthesis of the Ag(+)-inducible component(s) and rendered cells resistant to the toxic effects of Ag(+), suggesting competition between Cu(2+) and Ag(+) ions, possibly at the level of metal uptake.