Heavy-metal regulation of thioredoxin gene expression in Chlamydomonas reinhardtii

Heavy metals are highly toxic compounds for cells. In this report we demons trate that the expression of Chlamydomonas reinhardtii thioredoxins (TRX) m and h is induced by heavy metals. Upon exposure of the cells to Cd and Hg, a strong accumulation of both messengers was observed. Western-blot experi ments revealed that among these two TRXs, only TRX h polypeptides accumulat ed in response to the toxic cations. A biochemical analysis indicated that heavy metals inhibit TRX activity, presumably by binding at the level of th eir active site. Sequence analysis of the C. reinhardtii TRX h promoter rev ealed the presence of cis-acting elements related to cadmium induction. The origins and purposes of this regulation are discussed. Our data suggest, f or the first time to our knowledge, a possible implication of TRXs in defen se mechanisms against heavy metals.