ABSTRACT: Because iron toxicity and deficiency are equally life threatening, maintaining intracellular iron levels within a narrow optimal range is critical for nearly all known organisms. However, regulatory mechanisms that establish homeostasis are not well understood in organisms that dwell in environments at the extremes of pH, temperature, and salinity. Under conditions of limited iron, the extremophile Halobacterium salinarum, a salt-loving archaeon, mounts a specific response to scavenge iron for growth. We have identified and characterized the role of two transcription factors (TFs), Idr1 and Idr2, in regulating this important response. An integrated systems analysis of TF knockout gene expression profiles and genome-wide binding locations in the presence and absence of iron has revealed that these TFs operate collaboratively to maintain iron homeostasis. In the presence of iron, Idr1 and Idr2 bind near each other at 24 loci in the genome, where they are both required to repress some genes. In contrast, Idr1 and Idr2 are both necessary to activate other genes in a putative a feed forward loop. Even at loci bound independently, the two TFs target different genes with similar functions in iron homeostasis. We discuss conserved and unique features of the Idr1-Idr2 system in the context of similar systems in organisms from other domains of life. Data in this GEO archive are linked to the publication: Schmid AK, Pan M, Sharma K, Baliga NS.2011. Two transcription factors are necessary for iron homeostasis in a salt-dwelling archaeon.Nucleic Acids Res.39(7):2519-33. Cultures containing either the gene encoding the Idr1 or Idr2 transcription factors with c-terminal fusions to the myc epitope were grown to mid-logarithmic phase in the presence or absence of 100 uM FeSO4. Cultures were subjected to ChIP-chip as described in Facciotti, MT, Reiss, DJ, Pan, M, Kaur, A, Vuthoori, M, Bonneau, R, Shannon, P, Srivastava, A, Donohoe, SM, Hood, LE and Baliga, NS. General transcription factor specified global gene regulation in archaea. Proc Natl Acad Sci U S A. 2007;104: 4630-4635. Each Sample is based on two arrrays (one with dye-swap).
