SPATIAL REGULATION OF SPMTA METALLOTHIONEIN GENE-EXPRESSION IN SEA-URCHIN EMBRYOS BY A REGULATORY CASSETTE IN INTRON-1

The SpMTA metallothionein (MT) gene of the sea urchin Strongylocentrot us purpuratus is restricted in its expression to the aboral ectoderm i n gastrulae and pluteus larvae. The proximal 1.6 kb of the 5'-flanking region together with the 1.12-kb first intron of the SpMTA gene are s ufficient for its correct cell-type specific expression in transgenic embryos. This restricted spatial expression is largely eliminated by d eletion of an interior 405-bp region in the intron. Within this region is a 295-bp, genomically repetitive, transposon-like segment (Nemer e t al., 1993), containing several sequence motifs highly homologous to posited regulatory elements in the promoters of other genes (Thiebaud et al., 1990). The P3A and P5 sites in this apparent regulatory casset te were shown through competition to bind with relatively high affinit ies the same nuclear factors, bound by their counterpart sites in the CyIIIa actin promoter.