Cloning and characterization of a gene coding for a novel metallothionein in the Pacific oyster Crassostrea gigas (CgMT2): a case of adaptive response to metal-induced stress?

Cases of heavy metal resistance acquisition have already been demonstrated in eukaryotes, which involve metallothionein (MT) gene duplication or amplification mechanisms. We characterized in a marine bivalve, Crassostrea gigas, a gene coding for an unusual MT, which has never been described in other species. Our results illustrate a unique case of exon duplication and rearrangement in the MT gene family. The particular organization of the third exon of this gene allows the synthesis of a MT that presents a higher metal ion binding capacity compared to previously described MTs. The formation of a supplementary third structural beta-domain is proposed to explain results obtained in in vitro experiments. Differences in the metal responsive element (MRE) copy number and MRE core sequence observed in the promoter of CgMT2 also suggest differential regulation of CgMT2 transcription and possible implication in the detoxification processes.