Fig. 10. Schematic summary of the proposed mechanism of RirA-mediated regulation of Fe-responsive genes in Rhizobia. Under iron sufficiency (grey background), RirA binds a [4Fe–4S] cluster (made via the Suf system) and associates with the IRO motif (sequence shown) of the promoter regions of RirA-regulated genes, resulting in repression of transcription. RirA regulated genes include those indicated: tonB (energy transducer for iron uptake), hmu (heme uptake) vbs and fhu (for the synthesis and uptake of the vicibactin siderophore, respectively), suf (iron-sulfur cluster biosynthesis), irr (a second global iron regulator) and rirA. Under iron deficiency (white background), the [4Fe–4S] cluster of RirA initially loses a Fe²⁺ ion to form an unstable [3Fe–4S] cluster that degrades to a [2Fe–2S] form and eventually to apo RirA. Forms of RirA containing a degraded Fe–S cluster, or lacking a cluster entirely, bind the IRO motif with lower affinity, and may also be subject to degradation, resulting in increased expression. O₂/oxidative stress destabilises the RirA cluster, leading to increased turnover of [4Fe–4S] RirA even under iron sufficiency. Figure adapted from ref. 8.