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Molecular cloning of chicken metallothionein. Deduction of the complete amino acid sequence and analysis of expression using cloned cDNA.

A cDNA library was constructed using RNA isolated from the livers of chickens which had been treated with zinc. This library was screened with a RNA probe complementary to mouse metallothionein-I (MT), and eight chicken MT cDNA clones were obtained. All of the cDNA clones contained nucleotide sequences homologous to regions of the longest (376 bp) cDNA clone. The latter contained an open reading frame of 189 bp, and the deduced amino acid sequence indicates a protein of 63 amino acids of which 20 are cysteine residues. Amino acid composition and partial amino acid sequence analyses of purified chicken MT protein agreed with the amino acid composition and sequence deduced from the cloned cDNA. Amino acid sequence comparisons establish that chicken MT shares extensive homology with mammalian MTs, but is more closely related to the MT-II than to the MT-I isoforms from various mammals. The nucleotide sequence of the coding region of chicken MT shares approximately 70% homology with the consensus sequence for the mammalian MTs. Southern blot analysis of chicken DNA indicates that the chicken MT gene is not a part of a large family of related sequences, but rather is likely to be a unique gene sequence. In the chicken liver, levels of chicken MT mRNA were rapidly induced by metals (Cd2+, Zn2+, Cu2+), glucocorticoids and lipopolysaccharide. MT mRNA was present in low levels in embryonic liver and increased to high levels during the first week after hatching before decreasing again to the basal levels found in adult liver. The results of this study establish that MT is highly conserved between birds and mammals and is regulated in the chicken by agents which also regulate expression of mammalian MT genes. However, in contrast to the mammals, the results suggest the existence of a single isoform of MT in the chicken.