cDNA cloning of a cadmium-inducible mRNA encoding a novel cysteine-rich, non-metallothionein 25-kDa protein in an enchytraeid earthworm.

Cadmium accumulation and its effect on gene expression have been investigated at sublethal cadmium concentrations in the soil oligochaete Enchytraeus buchholzi. This worm is capable of accumulating cadmium to large amounts, which coincides with the induction of a mRNA isolated as a cDNA clone by differential screening of a cDNA library constructed from cadmium-treated enchytraeids. The cDNA clone designated CRP1 is 1474 base pairs in length and contains a 753-base pair open reading frame, encoding a novel Cys-rich non-metallothionein protein. In vitro translation of the in vitro transcribed CRP1 results in a protein with a molecular mass of 25 kDa and an pI of approximately 7.5. These values are consistent with those predicted from the deduced amino acid sequence. The CRP protein contains 27% Cys, most of them arranged in Cys-X-Cys and Cys-Cys segments. The sequence is also characterized by a 31-amino-acid motif, which is tandemly repeated along the sequence. Northern blot analysis reveals that the CRP gene is not constitutively expressed in untreated worms, but rather it is rapidly induced by cadmium. The CRP gene may be a promising candidate gene for monitoring bioavailable cadmium at subtoxic levels in terrestric environments.