Multiple Otx binding sites required for expression of the Strongylocentrotus purpuratus Spec2a gene

The control region of the aboral ectoderm-specific Spec2a gene of Strongylocentrotus purpuratus contains a 188-bp enhancer element, the RSR enhancer, required for temporal activation and aboral ectoderm/mesenchyme cell expression. Within the enhancer is a positive cis-regulatory element with the core consensus sequence TAATCC, which is capable of binding the sea urchin orthodenticle-related homeobox protein SpOtx. In this report, we extend our analysis of the RSR enhancer by dissecting it into smaller pieces and testing these pieces in an enhancer activation assay. The 188-bp enhancer region could not be divided without partial loss of activity, and two of the three pieces tested exhibited some activity. Using site-directed mutagenesis, we showed that three Otx consensus binding sites were responsible for the activity of the enhancer, acting in a non-cooperative manner to yield full activity. Mutagenizing the three Otx sites and a fourth one just upstream abolished all activity in the context of the complete Spec2a control region. Bandshift analysis revealed that the Otx sites were able to bind SpOtx, suggesting that this transcription factor mediates positive control at these sites. Non-SpOtx binding sites overlapping two of the Otx sites may also play a role in Spec2a expression. Using a lacZ reporter gene, we showed that a 76-bp DNA fragment containing two of the Otx sites was sufficient for aboral ectoderm/mesenchyme cell expression. These results suggest that the RSR enhancer plus an upstream DNA element required for mesenchyme cell repression are necessary and sufficient for the proper temporal activation and aboral ectoderm expression of the Spec2a gene and that the Otx elements play a positive role in this process.