FIGURE 10-33 Iron-dependent regulation of mRNA translation and degradation. The iron-response element–binding protein (IRE-BP) controls (a) translation of ferritin mRNA and (b) degradation of transferrin-receptor (TfR) mRNA. At low intracellular iron concentrations, IRE-BP binds to iron-response elements (IREs) in the 5′ or 3′ UTR of these mRNAs. At high iron concentrations, IRE-BP undergoes a conformational change and cannot bind either mRNA. The dual control by IRE-BP precisely regulates the level of free iron ions within cells. See the text for discussion.