Nucleotide sequence and expression of a Drosophila metallothionein

A Drosophila melanogaster cDNA clone was isolated based on its more intense hybridization to RNA sequences from copper-fed larvae than from control larval RNA. This clone showed strong hybridization to mouse metallothionein I cDNA at reduced stringency. Its nucleotide sequence includes an open reading segment which codes for a 40-amino acid protein; this protein is identified as metallothionein based on its similarity to the amino-terminal portion of mammalian and crab metalloproteins. The 10 cysteine residues present occur in five pairs of near vicinal cysteines (Cys-X-Cys). This cDNA sequence hybridized to a 400-nucleotide polyadenylated RNA whose presence in the cells of the alimentary canal of larvae was stimulated by ingestion of cadmium or copper; in other tissues this RNA was present at much lower levels. Mercury, silver, and zinc induced metallothionein to a lesser extent. The level of metallothionein RNA increased very soon after the initiation of metal treatment and reached a maximum after approximately 36 h.