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We have selected a metallothionein (MT)cDNA clone from a cadmium-resistant Drosophila melanogaster cell line. This clone includes an open reading frame coding for a 43-amino acid protein whose characteristics are a high cysteine content (12 cysteines, 28% of all residues) and a lack of aromatic amino acids. RNA from larvae and adults was extracted from batches of 50 individuals disrupted at 0C in 1 ml of 100 mM NaCl/10 mM EDTA/0.5% NaDodSO4/10 mM Tris HCl, pH 7.4, and then extracted with phenol/chloroform (7). RNA was precipitated at 40C by addition of LiCl to 4 M. The method of Aviv and Leder (8) was used to purify poly(A)+ RNA. Genomic DNA was prepared according to the method of Junakovic (9).In Vitro Translation. Both total cellular RNA and poly(A)+ RNA were translated in a wheat germ cell-free system as described by the supplier (Amersham).[35S]Cysteine (1000 Ci/mmol, New England Nuclear; 1 Ci = 37 GBq) was used as the radioactively labeled amino acid. After translation, the temperature was raised to 65°C for 10 min. Thermoresistant proteins were then carboxymethylated and electrophoresed on 8 M urea/0. 1% NaDodSO4/20% polyacrylamide gels (10).Construction and Screening of a Cd R200 cDNA Library. A cDNA library was constructed starting from 5 ,g of poly(A)+ RNA purified from Cd R200 cells (11). Insertion of the cDNA at the EcoRI site of vector plasmid pUC8 was performed after addition of EcoRI linkers (12). The protocol of Mandel and Higa (13) 2658The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

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