The Irr and RirA Proteins Participate in a Complex Regulatory Circuit and Act in Concert To Modulate Bacterioferritin Expression in Ensifer meliloti 1021

In this work we found that thebfr gene of the rhizobial speciesEnsifer meliloti , encoding a bacterioferritin iron storage protein, is involved in iron homeostasis and the oxidative stress response. This gene is located downstream of and overlapping thesmc03787 open reading frame (ORF). No well-predicted RirA or Irr boxes were found in the region immediately upstream of thebfr gene although two presumptive RirA boxes and one presumptive Irr box were present in the putative promoter ofsmc03787 . We demonstrate thatbfr gene expression is enhanced under iron-sufficient conditions and that Irr and RirA modulate this expression. The pattern ofbfr gene expression as well as the response to Irr and RirA is inversely correlated to that ofsmc03787 . Moreover, our results suggest that the small RNA SmelC759 participates in RirA- and Irr-mediated regulation ofbfr expression and that additional unknown factors are involved in iron-dependent regulation.IMPORTANCE E. meliloti belongs to theAlphaproteobacteria , a group of bacteria that includes several species able to associate with eukaryotic hosts, from mammals to plants, in a symbiotic or pathogenic manner. Regulation of iron homeostasis in this group of bacteria differs from that found in the well-studiedGammaproteobacteria . In this work we analyzed the effect ofrirA andirr mutations onbfr gene expression. We demonstrate the effect of anirr mutation on iron homeostasis in this bacterial genus. Moreover, results obtained indicate a complex regulatory circuit where multiple regulators, including RirA, Irr, the small RNA SmelC759, and still unknown factors, act in concert to balancebfr gene expression.