The mouse porphobilinogen deaminase gene. Structural organization, sequence, and transcriptional analysis

The porphobilinogen deaminase gene encodes the third enzyme of the heme biosynthetic pathway. This gene is expressed in a tissue-specific manner and gives rise to two isoenzymatic forms encoded by mRNA species differing in their 5' extremity. Recent studies in human demonstrated that the tissue-specific expression of the porphobilinogen deaminase gene is determined in erythropoietic cells, by the utilization of a specific promoter situated 3' to the housekeeping promoter used in other cell types. This results, through differential splicing, in the mutually exclusive presence of either exon 1 or exon 2 in mature mRNAs. Here, we report the cloning and sequencing of the porphobilinogen deaminase gene from mouse. The overall organization of the mouse gene is similar to that of the human one. In the housekeeping promoter, only a short stretch of homology is found including two potential Sp1 binding sites; in contrast, more extensive similarity appears in the erythroid-specific promoter including two motifs also found in globin gene, a CACCC box, and a recently described Ery F1 consensus binding sequence. We derived a set of single-stranded probes corresponding to different parts of the mouse gene to carry out a detailed analysis of the transcriptional unit in various cell types, using a run-on transcription assay on isolated nuclei. In liver cells, the first (non-erythropoietic) exon is more actively transcribed than parts of the gene situated downstream, suggesting that the elongation of transcripts is blocked within the 5' part of the first intron. In erythropoietic cells, the downstream promoter becomes activated; surprisingly, the initiation of transcription is also enhanced from the upstream (housekeeping) promoter and most of the transcripts initiated at the housekeeping promoter stop downstream of the first exon, between the two promoters.