Metallothionein mRNA Sequencing and Induction by Cadmium in Gills of the Crucian Carp, Carassius auratus

Abstract

Metallothioneins (MTs) are inducible metal-binding proteins characterized by low molecular weight (6000-10000 Da) and play a major role in detoxification of heavy metals. They are known to be induced by heavy metals in various organs of different species and represent a potential biomarker of aquatic contamination with heavy metals. In this study, cloning and sequencing of a MT gene of the Korean freshwater fish, the crucian carp (Carassius auratus), were performed. Furthermore, the gene expression and cadmium accumulation in the gills of the crucian carp were compared during the cadmium exposure period of 25 days. The MT gene, lacking the 5′ promoter region, is 405 bp long and has a tripartite structure consisting of three exons and two introns. The open reading frame encodes a cysteine-rich (20 cysteines) polypeptide of 60 amino acids containing six Cys-X-Cys motifs, typically found in this protein family. It was found that the expression of MT mRNA of gills was very low in normal control fish but high mRNA expression was induced after 1-day exposure to cadmium. However, the mRNA levels decreased after 1 day even though the cumulative cadmium concentration increased during the remaining exposure period. These results suggest that MT expression does not always reflect the cadmium accumulation in the gills of the fish and may have limitations in its use as a biomarker to monitor cadmium contamination of aquatic environments.