Identification of two metallothionein isoforms by molecular cloning of their cDNAs in fresh-water fish, crucian carp (Carassius-cuvieri)

Generally, there are two major isoforms of metallothionein (MT) in mammals. In this study two cDNAs of metallothionein, MT-A and MT-B, in a fresh-wate r fish, crucian carp (Carassius cuvieri), were cloned by the rapid amplific ation of cDNA ends (RACE). The homology of their reading frame is about 92. 3%, The sequence analysis of both cDNAs gave the structures of coding regio ns corresponding to 60 amino acid residues, and the structures of complete 3'-untranslated regions in which a significant difference in the size of th eir 3'-untranslated regions (130 bp for MT-A and 280 bp for MT-B) exists. T he results of amino acid sequencing of both MT-1 and MT-2 purified by HPLC are identical to those deduced from MT cDNA genes, indicating that MT-1 is from MT-A gene and MT-2 is from MT-B gene respectively. No blocking in the N-terminal of MT-2 isoform was the first case found in vertebrates, most of which were blocked by acetylation, These results suggest that there were d ifferential controls at the transcription level and after translation of th ese two MT isoforms. And this gives a clue to understand the diversities of their functions.