Inhibition of Heme Biosynthesis Prevents Transcription of Iron Uptake Genes in Yeast

Robert J Crisp,Annette Pollington,Charles Galea,Shulamit Jaron,Yuko Yamaguchi-Iwai,Jerry Kaplan

doi:10.1074/jbc.m307229200

Abstract

Yeast are capable of modifying their metabolism in response to environmental changes. We investigated the activity of the oxygen-dependent high-affinity iron uptake system of Saccharomyces cerevisiae under conditions of heme depletion. We found that the absence of heme, due to a deletion in the gene that encodes delta-aminolevulinic acid synthase (HEM1), resulted in decreased transcription of genes belonging to both the iron and copper regulons, but not the zinc regulon. Decreased transcription of the iron regulon was not due to decreased expression of the iron sensitive transcriptional activator Aft1p. Expression of the constitutively active allele AFT1-1up was unable to induce transcription of the high affinity iron uptake system in heme-depleted cells. We demonstrated that under heme-depleted conditions, Aft1p-GFP was able to cycle normally between the nucleus and cytosol in response to cytosolic iron. Despite the inability to induce transcription under low iron conditions, chromatin immunoprecipitation demonstrated that Aft1p binds to the FET3 promoter in the absence of heme. Finally, we provide evidence that under heme-depleted conditions, yeast are able to regulate mitochondrial iron uptake and do not accumulate pathologic iron concentrations, as is seen when iron-sulfur cluster synthesis is disrupted.

Highlights

The insolubility of Fe3ϩ in aqueous solution and the toxicity of iron-generated oxygen radicals has led to the assumption that the use of iron in biochemical reactions initially occurred in an anaerobic environment
Mitochondria are the site of two critical points in iron metabolism: the iron chelation step of heme biosynthesis, and Fe-S cluster synthesis
In humans with sideroblastic anemia, inhibition of heme biosynthesis has been shown to result in pathological iron accumulation in the mitochondria of erythroid precursor cells

Summary

Introduction

The insolubility of Fe3ϩ in aqueous solution and the toxicity of iron-generated oxygen radicals has led to the assumption that the use of iron in biochemical reactions initially occurred in an anaerobic environment. While many iron-requiring biochemical reactions in bacteria occur anaerobically, most eukaryotic iron-requiring biochemical reactions involve electronmediated oxygen binding or transfer Examples of these reactions include nucleic acid reduction, lipid desaturation, and sterol synthesis (1). Many of the genes that encode iron-dependent enzymes are regulated by heme-sensing transcriptional activators (Hap1p, Hap complex) or repressors (Rox1p, Mot3p) (3). In this communication we demonstrate that in the absence of heme, transcription of the iron and copper regulons is inhibited, while the zinc regulon is unaffected. We demonstrate that inhibition of heme biosynthesis in yeast prevents transcription of the high-affinity iron transport system, precluding mitochondrial iron accumulation

Methods

Results

Conclusion