Developmental utilization of SpP3A1 and SpP3A2: two proteins which recognize the same DNA target site in several sea urchin gene regulatory regions.

SpP3A1 and SpP3A2 are DNA-binding proteins that interact specifically with the same target sites in the regulatory domains of the Strongylocentrotus purpuratus CyIIIa gene and also of several other known genes. In this work we used antibodies raised against recombinant P3A1 and P3A2 to quantitate these transcription factors in eggs and in the nuclear compartments of embryos of various stages. Both proteins are present in unfertilized eggs, and both enter the embryonic nuclei early in development, but only P3A2 remains present in nuclei at functional concentrations beyond the early gastrula stage. Combined with earlier measurements of P3A site binding at cleavage stages, these measurements show that P3A1 would be replaced by P3A2 at target sites in genes regulated by these factors.