Practice question

Regulation of Delta expression in sea urchin micromeres involves double-negative gate that ensures spatial restriction. Global repressor HesC binds regulatory elements of Delta and skeletogenic genes like alx1, ets1, tbr, keeping them transcriptionally silent in most embryonic cells. In micromeres, nuclear beta-catenin activates Pmar1 transcriptional repressor that specifically represses HesC transcription. Removal of HesC protein derepresses Delta, permitting its expression exclusively in micromeres where Pmar1 present. Therefore HesC negatively regulates Delta expression outside micromeres, while Pmar1 negatively regulates HesC, and beta-catenin positively regulates Pmar1. GSK-three acts far upstream, not directly on Delta promoter.