Human δ-aminolevulinate dehydratase (ALAD) gene: Structure and alternative splicing of the erythroid and housekeeping mRNAs

Genomic clones containing human δ-aminolevulinate dehydratase (ALAD), the second enzyme in the heme pathway, were isolated, and the entire sequence was determined in both orientations (15,913 bp; GenBank Accession No. X64467). The gene contained two alternative noncoding exons, 1A and 1B, and 11 coding exons, 2- 12. Ten Alu-repetitive elements were within the gene, including an inverted repeat that may have resulted from gene conversion. The housekeeping transcript, which included exon 1A and not 1B, was identified in a human adult liver cDNA library, while an erythroid-specific transcript, which contained exon 1B and not 1A, was detected in a human K562 erythroleukemia cDNA library. The promoter region upstream of housekeeping exon 1A was GC-rich and contained three potential Sp1 elements and a CCAAT box. Further upstream, there were three potential GATA-1 binding sites and an API site. The promoter region upstream of erythroid- specific exon 1B had several CACCC boxes and two potential GATA- 1 binding sites. To assess the tissue-specific expression of exons 1A and 1B. HeLa and K562 cells were transduced with CAT constructs containing either exon 1A or 1B and their respective upstream promoter region. Two housekeeping CAT constructs, with 450 and 1400 bp upstream of exon 1A, were expressed at similar levels in HeLa cells, whereas the erythroid-specific construct, containing the entire 450-bp promoter region upstream of exon 1B, was not. In contrast, the housekeeping and erythroid constructs were both expressed in K562 cells. These findings demonstrate that the human ALAD gene contains two promoter regions that generate housekeeping and erythroid-specific transcripts by alternative splicing, analogous to the expression of the human hydroxymethylbilane synthase gene, which encodes the third enzyme of the heme biosynthetic pathway. The novel expression of housekeeping and erythroid-specific transcripts apparently evolved to ensure sufficient heme biosynthesis for the high- level tissue-specific production of hemoglobin required for life.