Paired-like homeodomain proteins Phox2a/Arix and Phox2b/NBPhox have similar genetic organization and independently regulate dopamine beta-hydroxylasegene transcription

The homeodomain transcription factors Arix/Phox2a and NBPhox/Phox2b play a role in the specification of the noradrenergic phenotype of central and per ipheral neurons. To better understand the functions of these two factors, w e have compared the genetic organization, chromosomal location, and transcr iptional regulatory properties of Arix and NBPhox. The gene structure is ve ry similar, with each gene containing three exons and two introns, extendin g a total of approximately 5 kb, Arix and NBPhox are unlinked in human and mouse genomes, NBPhox is located on human Chromosome 4p12 and mouse Chromos ome 5, while Arix is located on human Chromosome 11q13 and mouse Chromosome 7. Both proteins bind to three sites in the promoter proximal region of th e rat dopamine beta-hydroxylase gene (DBH). In vitro, Arix and NBPhox form DNA-independent multimers and exhibit cooperative binding to the DB1 regula tory element, which contains two homeodomain recognition sites. Both protei ns regulate transcription from the rat DBH promoter, and transcription is s ynergistically increased in the presence of the protein kinase A catalytic subunit (PKA) plus either Arix or NBPhox. The transcription factors exhibit similar concentration-dependent efficacies, and when they are coexpressed, transcription is stimulated to a value approximately equal to that seen wi th either factor alone. The N-terminal segment of Arix is essential for tra nscriptional regulatory activity, and this region bears 50% identity with N BPhox, suggesting a similar mechanism of transcriptional activation of the DBH gene. We conclude from this study that Arix and NBPhox exhibit indistin guishable and independent transcriptional regulatory properties on the DBH promoter.