The AadR-FixK hierarchy coordinates iron-responsive metabolism via Fur-family regulators in Rhodopseudomonas palustris TIE-1
Rhodopseudomonas palustris (Rp.) is a metabolically versatile environmental bacterium that flourishes across gradients of iron, oxygen, and light. The varying metabolic landscapes Rp. exploits have pressured it to evolve hierarchical regulatory networks, enabling tuneable expression in response to multiple input signals. However, many studies of regulation in this organism have used standard laboratory conditions: incandescent illumination and single-substrate growth, which do not fully engage the complexity of its regulatory networks and lack relevance to natural environments. Rp. can generate energy through light-powered iron oxidation via the PioABC system, and swathes of its anaerobic metabolism employ proteins using ferrous cofactors produced from bioavailable Fe(II). Rp. lacks the Fe(II)-sensing regulators found in related lineages, leaving it unclear how it balances expression of anaerobic metabolic pathways with sensed iron levels. Using what we term naturomimetic conditions, including growth with simultaneous provision of an organic carbon source and Fe(II), we uncovered a surprising link between oxygen-responsive and iron-responsive regulatory networks in Rp. FixK and AadR mediate the regulatory shift from aerobic to anaerobic metabolism, but, as we demonstrate here, they also orchestrate the response to Fe(II) by modulating a secondary network of Fur-family regulators. {Delta}aadR and {Delta}fixK strains were both defective in iron metabolism, and {Delta}aadR showed perturbed expression of all four Fur-family regulators, sometimes in entirely opposite directions. Our findings establish that iron regulation in Rp. occurs via a regulatory network integrating multiple environmental signals. Phylogenetic analyses suggest conservation of network components across several related evolutionary lineages, including Bradyrhizobium.