Abstract
Iron increases synthesis rates of proteins encoded in iron-responsive element (IRE)-mRNAs; metabolic iron (“free,” “labile”) is Fe
2+
. The noncoding IRE-RNA structure, approximately 30 nt, folds into a stem loop to control synthesis of proteins in iron trafficking, cell cycling, and nervous system function. IRE-RNA riboregulators bind specifically to iron-regulatory proteins (IRP) proteins, inhibiting ribosome binding. Deletion of the IRE-RNA from an mRNA decreases both IRP binding and IRP-independent protein synthesis, indicating effects of other “factors.” Current models of IRE-mRNA regulation, emphasizing iron-dependent degradation/modification of IRP, lack answers about how iron increases IRE-RNA/IRP protein dissociation or how IRE-RNA, after IRP dissociation, influences protein synthesis rates. However, we observed Fe
2+
(anaerobic) or Mn
2+
selectively increase the IRE-RNA/IRP
K
D
. Here we show: (
i
) Fe
2+
binds to the IRE-RNA, altering its conformation (by 2-aminopurine fluorescence and ethidium bromide displacement); (
ii
) metal ions increase translation of IRE-mRNA in vitro; (
iii
) eukaryotic initiation factor (eIF)4F binds specifically with high affinity to IRE-RNA; (
iv
) Fe
2+
increased eIF4F/IRE-RNA binding, which outcompetes IRP binding; (
v
) exogenous eIF4F rescued metal-dependent IRE-RNA translation in eIF4F-depeleted extracts. The regulation by metabolic iron binding to IRE-RNA to decrease inhibitor protein (IRP) binding and increase activator protein (eIF4F) binding identifies IRE-RNA as a riboregulator.