The induction of ferritin synthesis in circulating larval red blood cells

Ferritin gene organization: differences between plants and animals suggest possible kingdom-specific selective constraints

Ferritin, a protein widespread in nature, concentrates iron approximately 10(11)-10(12)-fold above the solubility within a spherical shell of 24 subunits; it derives in plants and animals from a common ancestor (based on sequence) but displays a cytoplasmic location in animals compared to the plastid in contemporary plants. Ferritin gene regulation in plants and animals is altered by development, hormones, and excess iron; iron signals target DNA in plants but mRNA in animals. Evolution has thus conserved the two end points of ferritin gene expression, the physiological signals and the protein structure, while allowing some divergence of the genetic mechanisms. Comparison of ferritin gene organization in plants and animals, made possible by the cloning of a dicot (soybean) ferritin gene presented here and the recent cloning of two monocot (maize) ferritin genes, shows evolutionary divergence in ferritin gene organization between plants and animals but conservation among plants or among animals; divergence in the genetic mechanism for iron regulation is reflected by the absence in all three plant genes of the IRE, a highly conserved, noncoding sequence in vertebrate animal ferritin mRNA. In plant ferritin genes, the number of introns (n = 7) is higher than in animals (n = 3). Second, no intron positions are conserved when ferritin genes of plants and animals are compared, although all ferritin gene introns are in the coding region; within kingdoms, the intron positions in ferritin genes are conserved. Finally, secondary protein structure has no apparent relationship to intron/exon boundaries in plant ferritin genes, whereas in animal ferritin genes the correspondence is high. The structural differences in introns/exons among phylogenetically related ferritin coding sequences and the high conservation of the gene structure within plant or animal kingdoms of the gene structure within plant or animal kingdoms suggest that kingdom-specific functional constraints may exist to maintain a particular intron/exon pattern within ferritin genes. In the case of plants, where ferritin gene intron placement is unrelated to triplet codons or protein structure, and where ferritin is targeted to the plastid, the selection pressure on gene organization may relate to RNA function and plastid/nuclear signaling.

Ferritin (mRNA, protein) and iron concentrations during soybean nodule development

To study how iron-rich nodules concentrate and store iron, ferritin (mRNA, protein) was analyzed in developing soybean nodules and compared to nitrogenase (mRNA/activity) and leghemoglobin (mRNA, protein, heme). Both ferritin mRNA and protein concentrations increased early in nodulation. Later in nodulation ferritin protein declined, in contrast to the mRNA, as nitrogenase (mRNA and activity) increased and leghemoglobin (mRNA and protein) accumulated. A precursor/product relationship between iron stored in ferritin and iron in nitrogenase or leghemoglobin is suggested. The uncoordinated changes in ferritin mRNA and protein during nodulation contrast with nitrogenase mRNA and nitrogenase activity suggesting possible translational and posttranscriptional effects on ferritin expression.

The iron regulatory region of ferritin mRNA is also a positive control element for iron-independent translation

The iron regulatory element (IRE) in the 5'-untranslated region of ferritin mRNA interacts with a specific regulator protein (P-90, IRE-BP, or FRP) to block translation. High cellular iron changes the IRE/P-90 interaction to relax the translational block and allow polyribosome formation. We now show that the IRE and base-paired flanking regions also enhance translation in the absence of P-90, explaining the high translational efficiency of deregulated ferritin mRNA observed previously. The effect of the IRE on translational efficiency was examined by comparing four sets of mRNAs: (1) +/- IRE in animal (frog) ferritin, regulated translationally by iron in vivo; (2) +/- animal IRE fused with plant (soybean) ferritin, regulated transcriptionally by iron in vivo; (3) repositioned IRE in animal ferritin; (4) mutated IRE in animal ferritin with G16A substitution, which decreases P-90 binding (negative control). The IRE region increased translational efficiency of both the animal ferritin and the heterologous IRE/soybean ferritin fusion mRNAs; the effect was observed in cell-free translation systems from either plants (wheat germ) or animals (rabbit reticulocyte). Repositioning the IRE further from the 5' cap eliminated positive control of translation. The single base mutation had no effect, indicating that positive and negative translational control involves different sections of the IRE region. Thus, the IRE region in ferritin mRNA encodes both positive translational control and, when combined with the regulator protein P-90, negative translational control.