δ-Aminolevulinic acid synthase 1 (ALAS1) is the first and rate-limiting enzyme in the heme biosynthesis. It has been well known that heme exerts a negative feedback control over the transcription of ALAS1 gene to maintain intracellular heme at appropriate level. To clarify the mechanisms by which heme regulates the expression of ALAS1, we examined the promoter activity of the gene and identified the heme-responsive element (HRE) located in the proximal promoter of the mouse ALAS1 gene. Reporter and EMSA assays revealed the sequence (GCGGGGGCG), as the site of repression by heme, at -301/-293bp of the ALAS1 promoter. Subsequently, EMSA and ChIP assays showed that a transcription factor, early growth response 1 (Egr-1) and its major corepressors, NAB1 and NAB2 were found to bind to the ALAS1-HRE, and these bindings increased dependent on the level of intracellular heme. When Egr-1 and NAB1 in combination were expressed in the cells, decreases of the level of ALAS1 mRNA and intracellular level of heme were observed. These results suggest that Egr-1-NABs complex is involved in the regulation of the transcription of ALAS1 by heme, leading to the regulation of the heme biosynthesis.
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