Direct Fe2+ Sensing by Iron-responsive Messenger RNA.Repressor Complexes Weakens Binding

Fe2+ is now shown to weaken binding between ferritin and mitochondrial aconitase messenger RNA noncoding regulatory structures ((iron-responsive element) (IRE)-RNAs) and the regulatory proteins (IRPs), which adds a direct role of iron to regulation that can complement the well known regulatory protein modification and degradative pathways related to iron-induced mRNA translation. We observe that the K-d value increases 17-fold in 5'-untranslated region IRE-RNA.repressor complexes; Fe2+, is studied in the absence of O-2. Other metal ions, Mn2+ and Mg2+ have similar effects to Fe2+ but the required Mg2+ concentration is 100 times greater than for Fe2+ or Mn2+. Metal ions also weaken ethidium bromide binding to IRE-RNA with no effect on IRP fluorescence, using Mn2+ as an O-2-resistant surrogate for Fe2+, indicating that metal ions bound IRERNA but not IRP. Fe2+ decreases IRP repressor complex stability of ferritin IRE-RNA 5-10 times compared with 2-5 times for mitochondrial aconitase IRE-RNA, over the same concentration range, suggesting that differences among IRE-RNA structures contribute to the differences in the iron responses observed in vivo. The results show the IRE-RNA.repressor complex literally responds to Fe2+, selectively for each IRE-mRNA.