All endodermal and mesenchymal cells of the sea urchin embryo descend from the vegetal plate, a thickened epithelium of approximately 50 cells arising at the early blastula stage. Cell types that derive from the vegetal plate are specified conditionally by inductive interactions with underlying micromeres, but the molecular details of vegetal-plate specification remain unresolved. In a search for regulatory proteins that have roles in vegetal-plate specification, a screen was performed to clone Krüppel/Krox-related genes from a Strongylocentrotus purpuratus embryo cDNA library. One newly identified clone, named SpKrox1, contained four zinc fingers and a leucine zipper domain. SpKrox1 expression was low in unfertilized eggs, increased severalfold to the early blastula stage and decreased between the early gastrula and pluteus stages. SpKrox1 mRNA was first seen in macromeres of 16-cell stage embryos and was restricted to cells of the developing vegetal plate thereafter. Vegetal-plate expression corresponded to a ring of cells around the blastopore and overlapped the expression patterns of other genes with potential roles in vegetal plate-specification. As the vegetal-plate cells invaginated into the blastopore, SpKrox1 expression was lost, suggesting that its role was not in endoderm differentiation per se but rather in the initial establishment of the vegetal plate.
The LpS1 beta gene of Lytechinus pictus is activated at the late cleavage stage about 12 hr after fertilization. LpS1 beta transcripts accumulate exclusively in aboral ectoderm lineages. LpS1 beta is thus a classic example of a gene whose expression is tightly controlled both temporally and spatially during early development. Previous studies on the LpS1 beta promoter identified two G-string DNA elements, one proximal and one distal to the LpS1 beta transcriptional start site, which bind to an ectoderm-enriched nuclear factor. In this report, we show that the ectoderm G-string factor binds to a G/C-rich region larger than the G-string itself and that the binding of the G-string factor requires sequences immediately downstream from the G-string. These downstream sequences are essential for full promoter activity. Two regions of 5'-flanking DNA are required for positive control of LpS1 beta, region I from base pairs -762 to -511, and region II, which includes the G/C-rich element, from base pairs -108 to -61. Region I also contains a mesenchyme cell repressor element. The results indicate that LpS1 beta expression is regulated positively in ectoderm cells and negatively in mesenchyme cells. Similar positive and negative control mechanisms regulate the expression of the related Spec genes of Strongylocentrotus purpuratus, but in this gene family the DNA elements are entirely different. We hypothesize that cis-regulatory elements are evolutionarily dynamic and that many different combinations of DNA elements are capable of given rise to aboral ectoderm-specific expression.
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