Brain-derived neurotrophic factor gene organization and transcription in the zebrafish embryo

The gene encoding zebrafish brain-derived neurotrophic factor (BDNF) was cloned from a PAC genomic DNA library. The entire transcription unit was contained in two independently isolated clones that together encompass 120 kb of genomic DNA. The intron/exon organization of the zebrafish gene was found to be identical to that of the mammalian gene but only one promoter has so far been identified. The associated 5' exon is 67% identical to exon 1c of the rat BDNF gene. A search of the 5' flank of the cloned promoter for sequence similarities with known transcription factor binding sites revealed potential AP-1, CREB, and SP1 binding sites. Fusion constructs containing the cloned promoter and 1.7 kb of 5' flank and an enhanced green fluorescent protein reporter that becomes membrane-anchored were injected into 1-8 cell stage embryos. Expression was seen in notochord, muscle, epithelial and endothelial cells of the 1-day-old embryo in consonance with the endogenous gene. These results demonstrate that the cloned promoter mediates cell-specific expression.