Iron-regulated ferritin synthesis in animals is dominated by translational control of stored mRNA; ironinduced transcription of ferritin genes, when it occurs, changes the subunit composition of ferritin mRNA and protein and is coupled to translational control. Ferritins in plants and animals have evolved from a common progenitor, based on the simi-

In animal cells specialized for iron storage, iron-induced accumulation of ferritin is known to result from a shift of stored mRNA from the ribonucleoprotein fraction to polysomes. Previous reports with bean leaves suggested that in plants iron induction of ferritin synthesis would result from a regulation at the transcriptional level (F van der Mark, F Bienfait, H van der Ende cultures, the mechanism of regulation of ferritin synthesis in response to iron does not result from recruitment of preexisfing mRNA. They confirm that in plant systems, ferritin synthesis results from increased transcription of the corresponding genes.

A murine interferon (IFN) a/(3 receptor was cloned from the IFN-sensitive L1210 cell line on the basis of its homology with the human receptor. A combination of methods that includes the screening of random-primed and oligo(dT)-primed cDNA libraries and polymerase chain reactions with a single-side specificity was used. At the amino acid level, the murine IFN-a/fi shows 46% identity with its human counterpart. Both human WISH cells presenting a low sensitivity to mouse IFN and a murine L1210 mutant subline that does not express the receptor have been stably transfected with the murine IFN-a/fB receptor. Whereas transfected human cells became sensitive to a limited number of mouse IFN-a/fi subtypes, the transfected murine L1210 mutant was found to be fully complemented and became sensitive to all mouse IFNa/(3 subtypes tested, including those that were not active on transfected human cells. These results strongly suggest that the receptor described here is implicated in the mediation of the activities of all murine IFN-ei/i3 subtypes.The interferon (IFN) a/,B family consists of not less than 20 related genes (1). In the mouse system, an IFN-a/f3 protein mixture is generally produced by cells infected by virus (2). The biological effects of these IFNs, including the inhibition of virus replication and the inhibition of cell growth, are mediated by a cell surface receptor (3, 4). In the murine system, the quantitative relationship between binding and biological activities ofIFN-a/fB has been only poorly studied; it is assumed that the general conclusions concerning the correlation between binding and activities of IFNs found in human (5-7) are also true for the murine system. A cDNA encoding a human IFN-a/,f receptor has already been cloned by an expression cloning strategy in which a mouse cell clone transfected with human DNA has been isolated on the basis of its acquired sensitivity to the human (13) and selected in medium containing G418 (GIBCO) at 1.25 mg/ml. The IFN-sensitive and -resistant mouse L1210 cell lines (9) have been recloned from the sensitive and resistant clones used by Aguet (3) and named S61 and R101, respectively. They were cultivated in RPMI 1640 medium supplemented with 10%6 fetal calf serum, transfected by the electroporation procedure (14) in a 0.4-cm diameter cuvette (300 V; 960 OF) with a Bio-Rad Gene Pulser, and selected with G418 at 1.5 mg/ml. General Methods. Standard procedures were as described (15) Howley et al. (19). Briefly, hybridization was at 420C in 1 M NaCl/0.1% Ficoll/0.1% polyvinylpyrrolidone/0.1% bovine serum albumin/5% dextran sulfate/S mM EDTA/1% SDS/25% (vol/vol) deionized formamide in 50 mM sodium phosphate (pH 7.2). Washing was at 420C in 1 M NaCl/5 mM EDTA/1% SDS/30% formamide in 50 mM sodium phosphate (pH 7.2).Abbreviations: IFN, interferon; nt, nucleotide(s).

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.

No abstract