STRUCTURE AND EXPRESSION OF THE GENE ENCODING RAT NONSPECIFIC FORM DELTA-AMINOLEVULINATE SYNTHASE

To understand the regulatory mechanisms controlling the heme biosynthetic pathway in animal liver, RNA blot hybridization analysis was used to examine the developmental stage-specific transcription of the gene encoding nonspecific form delta-aminolevulinate synthase (ALAS-N). The expression of the erythroid-specific delta-aminolevulinate synthase (ALAS-E) mRNA was also studied. The results demonstrated that, while ALAS-E is the key enzyme which supplies large quantities of heme for hemoglobin synthesis in fetal rat liver, ALAS-N functions to supply heme for the cytochrome P-450 system in fetal, newborn, and adult rat liver. ALAS-N was also suggested to work as a housekeeper gene to supply heme for respiratory cytochromes and other hemoproteins in various tissues. The structure and organization of the rat ALAS-N gene were next analyzed to study the molecular mechanisms regulating ALAS-N gene transcription. The ALAS-N gene was found to span more than 14 kb in the rat genome, encompassing eleven exons. The promoter region of the gene was found to contain several potential cis-acting regulatory elements, including motifs matching the TATA box sequence and the nuclear respiratory factor 1 binding sequence. The organization of the rat ALAS-N gene was determined to be quite similar to that of the ALAS-E gene in mouse; the mouse ALAS-E gene consists of eleven exons. This observation suggested that the ancestral gene for ALA synthase in animals was probably composed of eleven exons, and both the ALAS-N and ALAS-E genes were derived from this ancestral gene.