Transcript-Selective Translational Silencing by Gamma Interferon Is Directed by a Novel Structural Element in the Ceruloplasmin mRNA 3′ Untranslated Region

Abstract

Transcript-selective translational control of eukaryotic gene expression is often directed by a structural element in the 3′ untranslated region (3′-UTR) of the mRNA. In the case of ceruloplasmin (Cp), induced synthesis of the protein by gamma interferon (IFN-γ) in U937 monocytic cells is halted by a delayed translational silencing mechanism requiring the binding of a cytosolic inhibitor to the Cp 3′-UTR. Silencing requires the essential elements of mRNA circularization, i.e., eukaryotic initiation factor 4G, poly(A)-binding protein, and poly(A) tail. We here determined the minimal silencing element in the Cp 3′-UTR by progressive deletions from both termini. A minimal, 29-nucleotide (nt) element was determined by gel shift assay to be sufficient for maximal binding of the IFN-γ-activated inhibitor of translation (GAIT), an as-yet-unidentified protein or complex. The interaction was shown to be functional by an in vitro translation assay in which the GAIT element was used as a decoy to overcome translational silencing. Mutation analysis showed that the GAIT element contained a 5-nt terminal loop, a weak 3-bp helix, an asymmetric internal bulge, and a proximal 6-bp helical stem. Two invariant loop residues essential for binding activity were identified. Ligation of the GAIT element immediately downstream of a luciferase reporter conferred the translational silencing response to the heterologous transcript in vitro and in vivo; a construct containing a nonbinding, mutated GAIT element was ineffective. Translational silencing of Cp, and possibly other transcripts, mediated by the GAIT element may contribute to the resolution of the local inflammatory response following cytokine activation of macrophages.

ACKNOWLEDGMENTS

This work was supported by Public Health Service grants HL29582 and HL67725 from the National Heart, Lung and Blood Institute, National Institutes of Health (to P.L.F.), by an American Heart Association Predoctoral Fellowship, Ohio Valley Affiliate and a Cleveland State University doctoral dissertation research expense award (to P.S.), and by a Scientist Development Grant from the American Heart Association, National Affiliate (to B.M.).

We gratefully acknowledge Donna Driscoll of the Lerner Research Institute, Cleveland Clinic Foundation, for helpful discussions and Matthias Hentze of the European Molecular Biology Laboratory for the 15-LOX 3′-UTR construct.