Identification of in vivo Hox13-binding sites reveals an essential locus controlling zebrafish brachyury expression

During early embryogenesis, the vertebrate embryo extends from anterior to posterior because of the progressive addition of cells from a posteriorly localized neuromesodermal progenitor (NMp) population. An autoregulatory loop between Wnt and Brachyury/Tbxt is required for NMps to retain mesodermal potential and, hence, normal axis development. We recently showed that Hox13 genes help to support body axis formation and to maintain the autoregulatory loop, although the direct Hox13 target genes were unknown. Here, using a new method for identifying in vivo transcription factor-binding sites, we identified more than 500 potential Hox13 target genes in zebrafish. Importantly, we found two highly conserved Hox13-binding elements far from the tbxta transcription start site that also contain a conserved Tcf7/Lef1 (Wnt response) site. We show that the proximal of the two elements is sufficient to confer somitogenesis-stage expression to a tbxta promoter that, on its own, only drives NMp expression during gastrulation. Importantly, elimination of this proximal element produces shortened embryos due to aberrant formation of the most posterior somites. Our study provides a potential direct connection between Hox13 and regulation of the Wnt/Brachyury loop.