EVOLUTION OF AVIAN METALLOTHIONEIN - DNA-SEQUENCE ANALYSES OF THE TURKEY METALLOTHIONEIN GENE AND METALLOTHIONEIN CDNAS FROM PHEASANT AND QUAIL

The turkey metallothionein gene (tkMT) was isolated from a phage lambda-turkey genomic DNA library by virtue of high identity with chicken MT cDNA. The nucleotide sequences of the proximal 240 bp of the 5'-flanking region, of each of the three exons, and of the intron/exon boundaries were determined. Comparisons of the nucleotide sequences of the tkMT and cMT genes revealed (1) absolute conservation of intronic DNA immediately flanking each respective intron/exon boundary, (2) high conservation (95.6%) of exonic DNA encoding translated regions of the mRNA, and (3) high conservation (95%) of exonic DNA encompassing the putative transcription start point and polyadenylation signals. Sequence comparisons of the tkMT and cMT promoters regions near the TATA box revealed that both promoters contain a highly conserved proximal metal-responsive enhancer (MRE-enhancer) motif. The deduced amino acid sequence (63 amino acids) of tkMT was identical with that of cMT. In order to further explore the degree of conservation of the protein coding regions of avian MT genes, partial MT cDNAs from turkey, quail (qMT), and pheasant (pMT) were amplified using the reverse transcriptase-polymerase chain reaction (RT-PCR) and primers corresponding to the amino- and carboxyl-terminal coding regions of cMT mRNA. RT-PCR reaction products were cloned and the DNA sequences of multiple cDNA clones from each species were determined. The results suggest the existence of a single MT mRNA in zinc-treated liver from turkey and pheasant and the existence of a major and possibly a minor MT mRNA in quail. Comparisons of the nucleotide sequences of the predominant cMT, tkMT, qMT, and pMT cDNAs revealed exceptionally high identity (97%) and the deduced MT peptide sequences (residues 15-57) were identical. The data suggest that the nucleotide sequence of these avian MT cDNAs is evolving at a predictably slow rate, but that the amino acid sequence of the encoded protein is exceptionally well conserved.