Characterization of hypersensitive sites, protein-binding motifs, and regulatory elements in both promoters of the mouse porphobilinogen deaminase gene

Porphobilinogen deaminase, the third enzyme in the heme biosynthetic pathway, is encoded by a gene having two different promoters. Differential splicing of transcripts from the promoters yields two distinct mRNA species that are translated to give two isoforms of the protein. One isoform is ubiquitous, whereas the other is erythroid-specific. In this study, we have analyzed the gene regulatory elements that contribute to the tissue-specific promoter utilization of the mouse porphobilinogen deaminase gene. Six nuclear DNase I-hypersensitive sites were mapped in erythroid and nonerythroid cells, and four of these regions were further analyzed for in vitro nuclear protein-binding sites. The erythroid-specific promoter contains three erythroid nuclear factor GF-1-binding sites. The proximal GF-1-binding site, together with an adjacent duplicated CACCC motif, was sufficient to confer erythroid-specific expression in functional studies. Furthermore, as upstream gene sequences were shown to greatly increase promoter activity in erythroid cells, it suggests an upstream erythroid-specific enhancer may also be required for the up-regulation of the erythroid-specific promoter during erythropoiesis.