Bifunctional iron sensor that switches between 2 activities depending on iron availability. Iron deprivation, promotes its mRNA binding activity through which it regulates the expression of genes involved in iron uptake, sequestration and utilization. Binds to iron-responsive elements (IRES) in the untranslated region of target mRNAs preventing for instance the translation of ferritin and aminolevulinic acid synthase and stabilizing the transferrin receptor mRNA. Conversely, when cellular iron levels are high, binds a 4Fe-4S cluster which precludes RNA binding activity and promotes the aconitase activity, the isomerization of citrate to isocitrate via cis-aconitate. citrate = D-threo-isocitrate Binds 1 [4Fe-4S] cluster per subunit. Interacts (when associated with the 4Fe-4S) with FBXL5. Interacts with frataxin(81-210). Belongs to the aconitase/IPM isomerase family.