Developmental regulation,induction, and embryonic tissue specificity of sea urchin metallothionein gene expression

Metallothionein (MT) is shown to be present in sea urchin embryos on the basis of its characteristic properties as a small protein (6–7 Da) of extraordinarily high cysteine content, whose biosynthesis is readily induced by heavy metals. Induction by Zn²⁺ results in the accumulation of the cysteine-rich MT protein, a 0.8 kb MT mRNA and a 2.9 kb nuclear RNA. The amount of MT mRNA is regulated intrinsically through the course of embryogenesis to the pluteus stage: A maternal MT mRNA is poly(A)-deficient and is polyadenylated after fertilization. New MT mRNA begins to accumulate between the seventh and eighth cell cleavage, reaches a maximum at the mesenchyme blastula stage, decreases during gastrulation, and rises again in the early pluteus stage. “Animalizing” embryos with Zn²⁺ during early embryogenesis causes a sustained accumulation of MT mRNA to levels greater than 25 times the normal amount. MT mRNA is present in high amount in the ectoderm of the pluteus, but is barely detectable in the mesoderm-endoderm tissue fraction. Treatment of either the pluteus or its isolated tissue fractions with Zn²⁺ results in the induction of MT mRNA accumulation in the mesoderm-endoderm but not in the already MT mRNA-enriched ectoderm. Furthermore, differences in Zn²⁺ induction of the MT gene in the blastula and gastrula are consistent with a developmental pattern in which MT gene expression is maintained constitutively at a high level in the ectoderm and at a low level in the mesoderm-endoderm tissues, which are, however, preferentially inducible by Zn²⁺.