Effects of Heavy Metals on Drosophila Larvae and a Metallothionein cDNA

Drosophila melanogaster larvae reared on food containing radioactive cadmium retained over 80% of it, mostly in the intestinal epithelium. The majority of this radioactivity was associated with a soluble protein of less than 10,000 molecular weight. Synthesis of this cadmium-binding protein was induced by the metal as demonstrated by incorporation of radioactive cysteine. Most copper ingested by larvae was also found to associate with a low molecular weight, inducible protein, but some of it was found in an insoluble fraction. Zinc was unable to, or very inefficient at, binding or inducing the synthesis of a similar protein. A D. melanogaster cDNA clone was isolated based on its more intense hybridization to copies of RNA sequences from copper-fed larvae than from control larvae. 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 was identified as metallothionein based on its similarity to the amino-terminal portion of mammalian and crab metalloproteins. The ten 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. Whether (any of) the protein(s) discussed above correspond(s) to that coded by this RNA sequence has not yet been determined.ImagesFIGURE 2.FIGURE 6.FIGURE 9

The Drosophila Homolog of Mammalian Zinc Finger Factor MTF-1 Activates Transcription in Response to Heavy Metals

Metallothioneins (MTs) are short, cysteine-rich proteins for heavy metal homeostasis and detoxification; they bind a variety of heavy metals and also act as radical scavengers. Transcription of mammalian MT genes is activated by heavy metal load via the metal-responsive transcription factor 1 (MTF-1), an essential zinc finger protein whose elimination in mice leads to embryonic lethality due to liver decay. Here we characterize the Drosophila homolog of vertebrate MTF-1 (dMTF-1), a 791-amino-acid protein which is most similar to its mammalian counterpart in the DNA-binding zinc finger region. Like mammalian MTF-1, dMTF-1 binds to conserved metal-responsive promoter elements (MREs) and requires zinc for DNA binding, yet some aspects of heavy metal regulation have also been subject to divergent evolution between Drosophila and mammals. dMTF-1, unlike mammalian MTF-1, is resistant to low pH (6 to 6.5). Furthermore, mammalian MT genes are activated best by zinc and cadmium, whereas in Drosophila cells, cadmium and copper are more potent inducers than zinc. The latter species difference is most likely due to aspects of heavy metal metabolism other than MTF-1, since in transfected mammalian cells, dMTF-1 responds to zinc like mammalian MTF-1. Heavy metal induction of both Drosophila MTs is abolished by double-stranded RNA interference: small amounts of cotransfected double-stranded RNA of dMTF-1 but not of unrelated control RNA inhibit the response to both the endogenous dMTF-1 and transfected dMTF-1. These data underline an important role for dMTF-1 in MT gene regulation and thus heavy metal homeostasis