On the organisation of the regulatory region of the zebrafish deltaD gene.

DeltaD is one of the four zebrafish Delta homologues presently known. Experimental evidence indicates that deltaD participates in a number of important processes during embryogenesis, including early neurogenesis and somitogenesis, whereby the protein it encodes acts as a ligand for members of the Notch receptor family. In accordance with its functional role, deltaD is transcribed in several domains of mesodermal and ectodermal origin during embryogenesis. We have analysed the organisation of the regulatory region of the deltaD gene using fusions to the reporter gene gfp and germline transgenesis. Cis-regulatory sequences are dispersed over a stretch of 12.5 kb of genomic DNA, and are organised in a similar manner to those in the regulatory region of the Delta-like 1 gene of mouse. Germline transformation using a minigene comprising 10.5 kb of this genomic DNA attached to the 3' end of a full-length cDNA clone rescues the phenotype of embryos homozygous for the amorphic deltaD mutation after eight(AR33). Several genomic regions that drive transcription in mesodermal and neuroectodermal domains have been identified. Transcription in all the neural expression domains, with one exception, is controlled by two relatively small genomic regions, which are regulated by the proneural proteins neurogenin 1 and zash1a/b acting as transcriptional activators that bind to so-called E-boxes. Transcriptional control of deltaD by proneural proteins therefore represents a molecular target for the regulatory feedback loop mediated by the Notch pathway in lateral inhibition.