STRUCTURAL REQUIREMENTS OF IRON-RESPONSIVE ELEMENTS FOR BINDING OF THE PROTEIN INVOLVED IN BOTH TRANSFERRIN RECEPTOR AND FERRITIN MESSENGER-RNA POSTTRANSCRIPTIONAL REGULATION

The synthesis of both transferrin receptor (TfR) and ferritin is regulated post-transcriptionally by iron. This is mediated by iron responsive elements (IRES) in the 5′- and 3′-untranslated regions, respectively, of TfR and ferritin mRNAs. Although these IRES have different sequences, they both form a characteristic stem-loop. We used competttion assays and partial peptide mapping of UV-crosslinked ferritin and TfR IRE-protein complexes to show that the cytosolic protein binding to the ferritin 5′-IRE, the iron-responsive element binding protein (IRE-BP), also binds to TfR 3′-IRES. To identify the structural requirements necessary for RNA-protein binding, ferritin IRE RNAs were synthesized which contained altered secondary structures and base substitutions. Affinities of these RNAs for IRE-BP were assayed in RNA-protein binding gels. Substitutions disrupting base-pairing of the stem prevented IRE-BP binding. Substitutions which restored base-pairing also restored IRE-BP binding. We conclude that the IRE-BPbinds to both ferritin and TfR IREs and recognizes a particular IRE conformation. © 1990 Oxford University Press.