cDNA cloning and cadmium-induced expression of metallothionein mRNA in the zebra mussel Dreissena polymorpha.

Using pooled degenerate oligonucleotides inferred from the N-terminal amino acid sequence of Dreissena polymorpha metallothionein and a Cys-X-Cys motif characteristic for known metallothioneins, a 150-bp metallothionein-specific reverse transcription PCR product was generated. The PCR product was used to screen a Dreissena polymorpha cDNA library, and a complete metallothionein cDNA sequence from Dreissena was identified. Four clones with the identical sequence were detected, supporting the idea of a single metallothionein gene in Dreissena. The sequence contains a 141-bp 5' untranslated region and a 572-bp 3' untranslated region with two polyadenylation signals. The coding region spans 219 bp. The deduced amino acid sequence shows 21 cysteine residues present in the metallothionein-typical motifs. Induction studies were performed with 50 microg Cd2+/L for up to 16 days. The exposed mussels show a sevenfold higher metallothionein mRNA level compared with uninduced control mussels.