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Cis-Acting Transcriptional Repression Establishes a Sharp Boundary in Chordate Embryos

BMP Signaling and Transcription Bone morphogenetic protein signaling (BMP) is widely used for dorsoventral axis determination in animal embryos, which involves specific interactions between BMPs and their antagonists. In the genomes of many animals, the gene Pinhead is encoded as a neighbor of one of the BMP ligand genes, Admp (antidorsalizing morphogenetic protein). Working in ascidian tailbud-stage embryos, Imai et al. (p. 964) now show that Pinhead is an antagonist of ADMP and is important for the establishment of the sharp boundary of the ventral epidermis. Transcription of Pinhead disturbs transcription of Admp by a cis-acting mechanism, which may explain the mutually exclusive expression of these two functionally opposed genes. The conserved tandem arrangement of Pinhead and Admp genes is important for their mutually exclusive expression. The function of bone morphogenetic protein (BMP) signaling in dorsoventral (DV) patterning of animal embryos is conserved among Bilateria. In vertebrates, the BMP ligand antidorsalizing morphogenetic protein (Admp) is expressed dorsally and moves to the opposite side to specify the ventral fate. Here, we show that Pinhead is an antagonist specific for Admp with a role in establishing the DV axis of the trunk epidermis in embryos of the ascidian Ciona intestinalis. Pinhead and Admp exist in tandem in the genomes of various animals from arthropods to vertebrates. This genomic configuration is important for mutually exclusive expression of these genes, because Pinhead transcription directly disturbs the action of the Admp enhancer. Our data suggest that this dual negative regulatory mechanism is widely conserved in animals.