Structure and Expression of Chicken Metallothionein

Abstract

The chicken metallothionein (MT) cDNA and gene were cloned, and their nucleotide sequences determined. The cDNA clones encode a cysteine-rich protein of 63 amino acids which shares extensive structural homology with the mammalian MTs. Southern blot analyses of total genomic DNA, and cloned chicken DNA indicated that the MT gene is a unique gene sequence. The chicken MT gene is structurally homologous with the mammalian MT genes; consisting of three exons separated by two intervening sequences. The placement of the intervening sequences in the chicken gene is nearly identical with that in the mammalian MT genes. Levels of hepatic MT mRNA were rapidly induced by metal ions (Cd²⁺, Zn²⁺, Cu²⁺), glucocorticoids and lipopolysaccharide (LPS). MT mRNA was present in low levels in embryonic liver, but was inducible in ovo by injection of metal ions, glucocorticoids or LPS. Hepatic MT mRNA increased to high levels soon after hatching, before decreasing again to the basal levels found in adult liver. Levels of hepatic MT mRNA in hatched chicks were influenced by dietary metals. The results establish that the structure of the MT protein and gene has been highly conserved between birds and mammals, which suggests a functionally important role(s) for this protein.