Ribosomal Pausing and Scanning Arrest as Mechanisms of Translational Regulation from Cap-Distal Iron-Responsive Elements

Iron regulatory protein 1 (IRP-1) binding to an iron-responsive element (IRE) located close to the cap structure of mRNAs represses translation by precluding the recruitment of the small ribosomal subunit to these mRNAs. This mechanism is position dependent; reporter mRNAs bearing IREs located further downstream exhibit diminished translational control in transfected mammalian cells. To investigate the underlying mechanism, we have recapitulated this position effect in a rabbit reticulocyte cell-free translation system. We show that the recruitment of the 43S preinitiation complex to the mRNA is unaffected when IRP-1 is bound to a cap-distal IRE. Following 43S complex recruitment, the translation initiation apparatus appears to stall, before linearly progressing to the initiation codon. The slow passive dissociation rate of IRP-1 from the cap-distal IRE suggests that the mammalian translation apparatus plays an active role in overcoming the cap-distal IRE–IRP-1 complex. In contrast, cap-distal IRE–IRP-1 complexes efficiently repress translation in wheat germ and yeast translation extracts. Since inhibition occurs subsequent to 43S complex recruitment, an efficient arrest of productive scanning may represent a second mechanism by which RNA-protein interactions within the 5′ untranslated region of an mRNA can regulate translation. In contrast to initiating ribosomes, elongating ribosomes from mammal, plant, and yeast cells are unaffected by IRE–IRP-1 complexes positioned within the open reading frame. These data shed light on a characteristic aspect of the IRE-IRP regulatory system and uncover properties of the initiation and elongation translation apparatus of eukaryotic cells.