Metal ion-responsive transgenic Xenopus laevis as an environmental monitoring animal

We generated germ line-transgenic Xenopus laevis that monitors environmental heavy metal ions. Sperm nuclei were transduced with cDNA of enhanced green fluorescent protein (EGFP) driven by murine metallothionein-1 gene promoters and were microinjected into unfertilized eggs. The eggs developed to sexually matured adults. The transgenic tadpoles at the premetamorphic stage were reared in water containing Zn 2+ and Cd 2+ separately at the concentrations of 0.38–1.52 and 0.09–0.44 μM, respectively. These animals responded to Zn 2+ at as low as 0.38 μM and Cd 2+ at as low as 0.44 μM. The level of EGFP fluorescence emitted by tadpoles increased as the concentration increased up to 1.52 μM and the exposure time prolonged up to 120 h. The fluorescent response was much more sensitive to Cd 2+ than to Zn 2+. We concluded that these transgenic tadpoles are useful as an animal indicator of environmental heavy metal ions.