Structure and metal-regulated expression of the gene encoding Xenopus laevis metallothionein-A

To investigate the regulation of amphibian metallothionein(MT)-encoding genes, we have isolated and sequenced the XlMT-A gene encoding Xenopus laevis (Xl) MT-A. The gene displays an overall structure similar to that of mammalian MT, with three exons interrupted by two introns. The promoter region contains a typical TATA box and two metal regulatory elements (MRE) within the first 100 bp upstream from the transcription start point (tsp). The transition metal ion (Mc2+) inducibility of the promoter was studied by transient expression experiments in CV-1 African green monkey kidney cells, using different DNA fragments from the 5'-flanking region of XlMT-A fused to the bacterial cat reporter gene. The first 145 bp upstream from the tsp are sufficient to confer inducibility of cat by Cd2+. Constructs bearing only the most proximal MRE are not inducible by Me2+, thus suggesting that both MRE are required for Me2+ induction. Recognition sites for the transcription factors, AP-1 and AP-2, are located within the first 180 bp of the promoter region and these elements appear to be involved in controlling the constitutive basal level of expression of this MT.