Transcription factor Irr controls iron homeostasis in alpha-porteobacteria. This regulator from the Fur protein family was shown to interact with heme and thus it senses the physiological consequence of the iron availability rather than its concentration per se. Irr was originally identified in Bradyrhizobium japonicum as a repressor of the heme biosynthesis genes. Later on it was shown to act as an activator of multiple iron uptake genes. Comparative genomic analysis identified DNA binding motifs of Irr and reconstructed the Irr regulons in 5 taxonomic groups of alpha-proteobacteria .
Among the predicted Irr targets are various genes involved in iron uptake, storage, synthesis of iron cofactors, and iron-dependent enzymes. Despite the observed difference in the Irr regulon content, the consensus sequences of the Irr-binding sites are well-conserved in various lineages of alpha-proteobacteria.