Practice question

HesC is global transcriptional repressor containing basic helix-loop-helix Orange domain expressed ubiquitously early in sea urchin. It binds E-boxes in promoters of micromere-specific genes including Delta ligand and skeletogenic transcription factors alx1, ets1, tbr, dri. Repression keeps Delta silent outside micromeres. In micromeres, Pmar1 represses hesC gene, derepressing targets and allowing Delta-Notch signaling. Therefore HesC expression represses Delta expression, ensuring spatially restricted signaling. Overexpression of HesC eliminates Delta and skeleton, while HesC morpholino knockdown causes ectopic Delta throughout embryo, confirming direct negative regulation rather than regulation of beta-catenin or micromere formation itself.