Evidence for a mesodermal embryonic regulator of the sea urchin CyIIa gene

The CyIIa gene of the sea urchin embryo is a model for study of cis-regulat ion downstream of cell-type specification, as CyIIa transcription follows t he specification and initial differentiation of the embryonic domains in wh ich it is expressed. These are the skeletogenic and secondary mesenchyme an d gut. We carried out a detailed structural and functional analysis of a ci s-regulatory region of this gene, extending 780 bp upstream and 125 bp down stream of the transcription start site, that had been shown earlier to repr oduce faithfully the complex and dynamic CyIIa pattern of expression. This analysis revealed that the overall pattern of expression of the CyIIa gene appears to be governed mainly by two independent sets of DNA elements, whic h are target sites for specific proteins present in blastula-stage nuclear extract. One type of element, which controls a dynamic program of expressio n in both skeletogenic and secondary mesenchyme cells, contains the consens us-binding site for a member of the ets transcription factor family. The ot her, which is responsible for the terminal or permanent phase of CyIIa expr ession in the gut, shares homologies with the late module of the endoderm-s pecific Endo16 gene (endo16 Module B). Oligonucleotides containing replicas of these two target sites fused upstream of a sea urchin basal promoter ar e sufficient to confer accurate mesenchyme and late gut expression of an in jected GFP construct. The finding of a single protein target site that reca pitulates CyIIa expression in both primary and secondary mesenchyme cells s uggests the existence of a pan-mesodermal gene expression program in the se a urchin embryo. (C) 2001 Academic Press.