Fig. 8. RirA mediated regulation of iron-responsive genes in Rhizobium. Under iron sufficient conditions, RirA accommodates a [4Fe–4S] cluster (via the Suf system) and binds to the IRO motif (sequence shown) present in the promoter regions of RirA-regulated genes, repressing their transcription. In low iron medium, the [4Fe–4S] cluster of RirA is unstable to conversion/degradation, yielding apo-RirA via a [2Fe–2S] form. Apo-RirA does not bind the IRO motif and so these genes are no longer repressed by RirA. [2Fe–2S] RirA retains some ability to (weakly) bind the IRO motif, resulting in a partial alleviation of repression under conditions of mild iron deficiency. O2/oxidative stress destabilizes the RirA cluster, leading to increased turnover of [4Fe–4S] RirA even under iron-replete conditions. We note that specific, as yet unidentified, cellular factors might also be involved in, and influence the rate of, cluster degradation.
