In Fungi

The Cu-induced metallothionein of S. cerevesiae (MW 6573) has gained a great deal of attention, while a copper-inducible metallothionein has also been reported from Neurospora crassa. This yeast protein is inducible only by copper, not zinc or cadmium, and is called Cu-MT or yeast MT. Inducible cadmium-binding proteins that are structurally different from Cu-MT have been isolated from Schizosaccharomyces pombe. These proteins are termed phytochelatins (sulfur-rich, metal-binding polypeptides). These phytochelators consist of three amino acids - cystine, glutamic acid, and glycine - that are analogs to similar peptides found in plant cells exposed to heavy metals such as Cd, Cu, Hg, Pb, and Zn. Multiple enzymes may be involved in phytochelatin synthesis, and so there must be mechanisms for the metal-dependent induction or activation of such enzymes. A low molecular weight Cd-binding protein has been isolated from Cd-resistant strains of S. cerevisiae. Yeast MT has been found in Cu-tolerant S. cenedesmus, and Cd induced metallothionein-like proteins in Chlorella pyrenoidosa and Dunaliella. A metallothionein protein has been identified in the ciliate Tetrahymena pyriformis after exposure to Cd, and this is also capable of Zn binding.

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