Multiple red cell ferritin mRNAs, which code for an abundant protein in the embryonic cell type, analyzed by cDNA sequence and by primer extension of the 5'-untranslated regions

Abstract

Ferritin maintains iron in a bioavailable, nontoxic form for vertebrates and invertebrates, higher plants, fungi, and bacteria; the protein is formed from two classes of subunits (H and L) in ratios which vary in different cell types. Ferritin may be an abundant, differentiation-specific protein or a "housekeeping" protein. The red cells of embryos are specialized for iron storage and have abundant ferritin; iron regulates the synthesis of ferritin in such cells translationally by recruitment of stored, ferritin mRNA and by translational competition. To characterize mRNA regulated in such a manner, we prepared cDNA from reticulocytes of bullfrog tadpoles, a readily available source of embryonic red cells; moreover, no protein sequence information was available for nonmammalian ferritin. An almost full-length (817 base pairs) cDNA (pJD5F12) was isolated and sequenced, the 5' end was analyzed by primer extension, and the cloned DNA was used as a hybridization probe. We have shown that ferritin mRNA is stored in the cytoplasm and that the 5' end of the mRNA is heterogeneous. The 5'-untranslated region of ferritin mRNA consisted of 143 nucleotides in the major (65%) species and 146 or 152 in the minor species (approximately 17% each). (Heterogeneity is characteristic of some other abundant mRNAs, e.g. globin, which is also translationally regulated.) Since excess iron had no detectable effect on the heterogeneity of the 5' end of ferritin mRNA, the feature is more likely associated with mRNA abundance and/or cell specialization than translational control. In the bullfrog, as in humans and rats, ferritin is encoded by multiple genomic sequences (four to eight) which specify proteins of considerable homology. For example, 75 of the 81 amino acids present in all mammalian ferritins sequenced are also present in the frog; the overall homology between frogs and humans or rats is 59-66%. Ferritin H and L subunits in humans are distinct (overall homology 56%) and appear to have diverged from a common precursor relatively recently. In contrast, ferritin H and L subunits have high homology in tadpole red cells, determined by hybrid select translation, which suggests that bullfrog red cell ferritin may be close to the primordial sequence.