Cis-regulatory architecture of a brain-signaling center predates the origin of chordates

Genomic approaches have predicted hundreds of thousands of tissue specific cis-regulatory sequences, but the determinants critical to their function and evolutionary history are mostly unknown1-4. Here, we systematically decode a set of brain enhancers active in the zona limitans intrathalamica (zli), a signaling center essential for vertebrate forebrain development via the secreted morphogen, Sonic hedgehog (Shh)5,6. We apply a de novo motif analysis tool to identify six position-independent sequence motifs together with their cognate transcription factors that are essential for zli enhancer activity and Shh expression in the mouse embryo. Using knowledge of this regulatory lexicon, we discover novel Shh zli enhancers in mice, and a functionally equivalent element in hemichordates, indicating an ancient origin of the Shh zli regulatory network that predates the chordate phylum. These findings establish a paradigm for delineating functionally conserved enhancers in the absence of overt sequence homologies, and over extensive evolutionary distances. Gene expression profiles from the mouse zona limitans intrathalamica (ZLI) region at E10.5