From: Yuh et al. (1998) Science 279, 1896-1902. Endo16 regulatory system of the sea urchin. (A) Diversity of protein binding sites and organization into modular subregions. Specific DNA binding sites are indicated as red blocks; modular subregions are denoted by letters G to A (Bp, basal promoter). Proteins binding at the target sites considered in this work are indicated: Otx, SpOtx-1; SpGCF1; the proteins CG, Z and P, which are not yet cloned; and protein C [ a CREB family protein] in subregion F. Proteins for which sites occur in multiple regions of the DNA sequence (indicated by the black line) are shown beneath. (B) Sequence of module A and location of protein binding sites. Sites are indicated in the same colors as in (A). A fragment containing CG3 and CG4 sites as well as Bp has no endoderm-specific activity and services other upstreams cis-regulatory systems promiscuously; similarly, the Endo16 cis-regulatory system functions specifically with heterologous promoters substituted for Bp. Box sequences indicated conserved core elements of the target sites, not the complete target site sequences. (C) Integrative and interactive functions of module A. Module A communicates the output of all upstream modules to the basal transcription apparatus. It also initiates endoderm expression, increases the output of modules B and G, and is required for functions of the upstream modules F, E, and DC. These functions are repression of expression in nonendodermal domains and enhancement of expression in response to LiCl.