A G/C-Rich DNA-Regulatory Element Controls Positive Expression of the Sea UrchinLytechinus pictusAboral Ectoderm-SpecificLpS1Gene

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Abstract
The LpS1β gene of Lytechinus pictus is activated at the late cleavage stage about 12 hr after fertilization. LpS1β transcripts accumulate exclusively in aboral ectoderm lineages. LpS1β is thus a classic example of a gene whose expression is tightly controlled both temporally and spatially during early development. Previous studies on the LpS1β promoter identified two G-string DNA elements, one proximal and one distal to the LpS1β 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 LpS1β, 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 LpS1β 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.