Uroporphyrinogen III synthase - An alternative promoter controls erythroid-specific expression in the murine gene

Uroporphyrinogen III synthase (URO-synthase, EC 4.2.1.75) is the fourth enz yme of the heme biosynthetic pathway and is the defective enzyme in congeni tal erythropoietic porphyria. To investigate the erythroid-specific express ion of murine URO-synthase, the cDNA and similar to 24-kilobase genomic seq uences were isolated and characterized. Three alternative transcripts were identified containing different 5'-untranslated regions (5'-UTRs), but iden tical coding exons 2B through 10. Transcripts with 5'-UTR exon IA alone or fused to exon 1B were ubiquitously expressed (housekeeping), whereas transc ripts with 5'-UTR exon 2A were only present in erythroid cells (erythroid-s pecific). Analysis of the TATA-less housekeeping promoter upstream of exon 1A revealed binding sites for ubiquitously expressed transcription factors Spl, NF1, AP1, Oct1, and NRF2. The TATA-less erythroid-specific promoter up stream of exon 2A had nine putative GATA1 erythroid enhancer binding sites. Luciferase promoter/reporter constructs transfected into NIH 3T3 and mouse erythroleukemia cells indicated that the housekeeping promoter was active in both cell lines, while the erythroid promoter was active only in erythro id cells. Site-specific mutagenesis of the first GATA1 binding site markedl y reduced luciferase activity in K562 cells (<5% of wild type). Thus, house keeping and erythroid-specific transcripts are expressed from alternative p romoters of a single mouse URO-synthase gene.