Abstract: Budding yeast responds to iron deprivation both by Aft1-Aft2-dependent transcriptional activation of genes involved in cellular iron uptake and by Cth1-Cth2-specific degradation of certain mRNAs coding for iron-dependent biosynthetic components. Here, we provide evidence for a novel principle of iron-responsive gene expression. This regulatory mechanism is based on the modulation of transcription through the iron-dependent variation of levels of regulatory metabolites. As an example, the LEU1 gene of branched-chain amino acid biosynthesis is down-regulated under iron-limiting conditions through depletion of the metabolic intermediate alpha-isopropylmalate which functions as key transcriptional co-activator of the Leu3 transcription factor. Synthesis of alpha-isopropylmalate involves the iron-sulfur protein Ilv3 which is inactivated under iron deficiency. As another example, decreased mRNA levels of the cytochrome c-encoding CYC1 gene under iron-limiting conditions involve heme-dependent transcriptional regulation via the Hap1 transcription factor. Synthesis of the iron-containing heme is directly correlated with iron availability. Thus, the iron-responsive expression of genes that are down-regulated under iron-limiting conditions is conferred by two independent regulatory mechanisms; transcriptional regulation through iron-responsive metabolites and post-transcriptional mRNA degradation. Only the combination of both processes provides a quantitative description of the response to iron deprivation in yeast.