Metallothionein gene expression in embryos of the sea urchin Lytechinus pictus

The metallothionein (MT) gene LpMT1 of the sea urchin Lytechinus pictus was characterized. The primary transcript of 3042 nucleotides includes four exons, as uniquely observed for other sea urchin MT genes, which are spliced to form a messenger RNA of 605 nucleotides. The deduced LpMT1 protein sequence includes 69 amino acids, more than observed for other MT proteins. For a high level of inducible activity, the LpMT1 promoter requires sequence elements in addition to the canonical regulatory elements identified for mammalian MT promoters. The promoter of the closely related LpMT2 gene is very active in spite of its lack of a distinctive poly(C) element included in a sequence tract required for fully induced activity of the LpMT1 promoter. In contrast to embryos of the sea urchin S. purpuratus in which MT mRNAs are restricted to the aboral ectoderm of uninduced embryos, no spatially preferential accumulation of MT mRNAs in L. pictus embryos was observed. The cisacting regulatory elements required for MT gene activity and the spatial specificity of MT gene expression in sea urchin embryos are considered. The LpMT1 and LpMT2 promoters constitute promiscuous promoters that can be induced to a high level of activity.