Requirements for the Translational Repression of Ferritin Transcripts in Wheat Germ Extracts by a 90-kDa Protein from Rabbit Liver*

Abstract A specific repressor of ferritin mRNA translation originally detected in rabbit reticulocyte lysates has now been purified to homogeneity from rabbit liver, as described in a companion paper (Walden, W. E., Patino, M. M., and Gaffield, L. (1989) J. Biol. Chem. 264, 13765-13769). This repressor is a 90-kDa protein that binds to a sequence in the 5'-untranslated region of ferritin mRNA. In this communication we describe the molecular features of a ferritin light chain transcript that are required for the repression of its translation by this protein. Addition of small amounts of the 90-kDa ferritin repressor protein (FRP) completely inhibited translation of ferritin transcripts in a wheat germ system. This repression did not require mRNA sequences contained in the 3'-untranslated region or in the majority of the ferritin coding region. In contrast, the first 130 nucleotides of the 5'-untranslated region, which contains the 28-nucleotide "iron responsive element" (IRE), was required for the repressive effect. Moreover, repression of full length transcripts was relieved by addition of a molar excess of a 92-nucleotide transcript of the 5'-untranslated region which also contained the IRE. These results suggest that no sequence information other than a portion of the 5'-untranslated region containing the IRE sequence is required for action of the 90-kDa FRP. In addition, a quantitative comparison of the repression of transcript with that of poly(A+) RNAs indicates that no post-transcriptional modifications of the latter (other than cap addition) are involved in the action of the 90-kDa FRP.