Structure, turnover, and heme-mediated suppression of the level of mRNA encoding rat liver δ-aminolevulinate synthase

λgt11 cDNA Libraries were constructed with poly(A)⁺ RNA preparations from both porphyric chicken and rat livers. A cDNA which encodes chicken hepatic δ-aminolevulinate synthase was cloned by screening with an anti-chicken liver δ-aminolevulinate synthase antibody. Using this cDNA as a probe, cDNAs encoding the entire protein coding sequence of rat hepatic δ-aminolevulinate synthase were then cloned. The complete nucleotide sequences of the cDNAs have been determined. The result predicts that the rat hepatic pre-δ-aminolevulinate synthase comprises 642 amino acids. We measured the half-life of the hepatic δ-aminolevulinate synthase mRNA by RNA blot hybridization analysis using allylisopropylacetamide-induced porphyric rats as an experimental model and the rat cDNA as a hybridization probe. The half-life of the mRNA determined by the injection of α-amanitin is as short as 20 min. This value is significantly shorter than the estimated half-lives of most other mRNAs in the differentiated tissues of animals. The effect of hemin administration on the level of hepatic δ-aminolevulinate synthase mRNA was also examined. The half-disappearance time of the mRNA after the hemin administration was essentially the same as that determined by α-amanitin or actinomycin D, and no additive effect was observed between α-amanitin and hemin on the half-life determination. The results provide convincing evidence that heme inhibits the transcription of δ-aminolevulinate synthase mRNA.