CIS-REGULATORY ELEMENTS REQUIRED FOR A LINEAGE-SPECIFIC GENE-EXPRESSION IN THE SEA-URCHIN EMBRYO

The sea urchin CyIIIa cytoskeletal actin gene is activated in late cleavage in blastomeres of the aboral ectoderm lineages, and throughout embryogenesis CyIIIa transcription is restricted to the aboral ectoderm. Temporal and spatial control is accomplished via a complex cis-regulatory domain extending approximately 2.3 kb upstream of the CyIIIa transcription initiation site. This regulatory domain includes some twenty sites of in vitro DNA:protein interaction. The functional significance in vivo of several of these elements has been established in transgenic embryos bearing CyIIIa.CAT (chloramphenicol acetyl transferase) reporter gene constructs, together with molar excesses of one of various nonoverlapping competitor sequences containing one, or a few, specific site(s) of in vitro interaction. The excess sites competitively titrated the available regulatory proteins away from the corresponding sites associated with the CyIIIa.CAT reporter genes. Competition with each of six nonoverlapping regions identified six different sites that function as positive regulators of CyIIIa transcription. Two additional regions identified nonhomologous sites that control spatial expression.