Information relay from gene to protein: the mRNP connection

Translational recruitment of Xenopus maternal mRNAs in response to poly(A) elongation requires initiation factor eIF4G-1

Xenopus oocytes accumulate maternal mRNAs which are then recruited to ribosomes during meiotic cell cycle progression in response to progesterone and coincident with poly(A) elongation. Prior to stimulation, most protein synthesis ( approximately 70%) does not require intact translation factor eIF4G (B. D. Keiper and R. E. Rhoads, 1997, Nucleic Acids Res. 25, 395-402). In the present study we have addressed the requirement of eIF4G in the recruitment of mRNAs during meiosis. Cleavage of eIF4G by coxsackievirus protease 2A inhibited progesterone-induced meiotic progression in 88% of the oocytes; prevented the recruitment of maternal mRNAs encoding cyclin B1, c-Mos, D7, and B9; and disrupted the association of eIF4G with poly(A)-binding protein. Poly(A) elongation, however, was not inhibited by eIF4G cleavage. Injection of MPF restored meiotic cell cycle progression to >60% of the oocytes but not the recruitment of cyclin B1 or B9 mRNA. Previously recruited maternal mRNAs were removed from polyribosomes following subsequent cleavage of eIF4G, indicating that eIF4G is required both to recruit and also to maintain maternal mRNAs on polyribosomes. The expression of a cleavage-resistant variant of human eIF4G-1 (G486E) significantly restored the ability to synthesize c-Mos in response to progesterone and to translate exogenous beta-globin mRNA, indicating that the inhibition by protease 2A is due to cleavage of eIF4G alone. These results indicate that intact eIF4G is required for the poly(A)-dependent recruitment of several maternal mRNAs (cyclin B1, c-Mos, D7, and B9) during meiotic cell cycle progression but not for the synthesis of most proteins.

Nuclear history of a pre-mRNA determines the translational activity of cytoplasmic mRNA

The pathways of synthesis and maturation of pre-messenger RNA in the nucleus have a direct effect on the translational efficiency of mRNA in the cytoplasm. The transcription of intron-less mRNA in vivo directs this mRNA towards translational silencing. The presence of an intron at the 5' end of the transcript relieves this silencing, whereas an intron at the 3' end further represses translation. These regulatory events are strongly dependent on the transcription of pre-mRNA in the nucleus. The impact of nuclear history on regulatory events in the cytoplasm provides a novel mechanism for the control of gene expression.

Isolation and characterization of goldfish Y box protein, a germ-cell-specific RNA-binding protein

Cyclin B is a regulatory subunit of maturation-promoting factor. In goldfish (Carassius auratus) oocytes, cyclin B is synthesized de novo after stimulation by 17alpha,20beta-dihydroxy-4-pregnen-3-one (maturation-inducing hormone). In this study, we examined goldfish oocyte proteins bound to cyclin B mRNA. Using oligo(dT)-cellulose affinity chromatography and northwestern blotting analysis, we identified a 54-kDa cyclin B mRNA-binding protein (p54). Southwestern blotting analysis showed the binding of p54 to the Y box DNA element (CTGATTGGCCAA), suggesting that p54 is a Y box protein in goldfish. We isolated two cDNA clones, GFYP1 and GFYP2, the latter of which encodes a germ-cell-specific Y box protein. An antibody against a GFYP2 protein recognized p54, suggesting that p54 is identical or highly similar to GFYP2 protein. This is also supported by the finding that a recombinant GFYP2 expressed in bacteria bound to both the Y box DNA element and the goldfish cyclin B mRNA synthesized in vitro. These results suggest that p54 is a germ-cell-specific Y box protein and is a potential masking protein of cyclin B mRNA in goldfish oocytes.

Overexpression in COS cells of p50, the major core protein associated with mRNA, results in translation inhibition

p50, the major core protein of messenger ribonucleoprotein particles (mRNPs) in the cytoplasm of somatic mammalian cells, has been characterized previously as a member of the Y-box binding transcription factor family of proteins (YB-protein) by both high structural homology and ability to bind specifically the Y-box sequence in double-stranded DNA. YB proteins are present in a whole range of cell types and some have been identified as germ-specific cytoplasmic proteins masking stored mRNA from translation. Western blot analysis of the distribution of p50 in subcellular fractions of COS-1 cells shows that p50 is a cytoplasmic protein quantitatively associated with mRNA, both in polyribosomes and in free mRNPs. The level of p50 in COS-1 cells determined by Western immunoblotting is 0.10% of total protein, which is nearly equimolar to that of ribosomes and is approximately 5-10-fold higher than the mRNA level. Transient transfection of COS-1 cells with a p50-expressing vector results in a dramatic inhibition of protein synthesis. A control transfection with a vector expressing a frameshift mutant of p50 does not cause translation inhibition. Therefore the increase in p50 protein level is responsible for the inhibitory effect in these cells.

Translational control of poly(A)-binding protein expression

Poly(A)-binding protein (PABP) is important for translation of eukaryotic mRNA and may be involved in shortening of its poly(A) tract. In many eukaryotic cells, this mRNA is inefficiently translated. The 5' untranslated region (UTR) of PABP mRNA has several adenine-rich regions which may serve as the PABP-binding sites to control its translation by a feed-back mechanism. This postulate was tested by using in vitro transcribed PABP mRNA and a rabbit reticulocyte lysate cell-free system. Results of our studies show that removal of the putative PABP-binding sites from the 5' UTR of this mRNA enhances its translation in the rabbit reticulocyte cell-free system. Furthermore, in vitro translation of the full-length PABP mRNA was inhibited by addition of purified PABP to the cell-free system. In contrast, translation of truncated mRNA lacking the putative PABP-binding sites at the 5' UTR was not inhibited by exogenous PABP. We have also tested the ability of purified PABP to bind to the 5' UTR of PABP mRNA using ultraviolet-mediated covalent cross-linking of RNA and proteins in vitro. Our results show that exogenous PABP binds to the 5' UTR of its full-length mRNA. Furthermore, incubation of PABP mRNA in rabbit reticulocyte lysate also led to binding of the endogenous PABP within the first 223 nucleotides of the 5' UTR. The adenine-rich regions are located within this segment of PABP mRNA. Following incubation of PABP mRNA in the reticulocyte lysate cell-free system under conditions of mRNA translation, the polysomal and non-translated free mRNA fractions were separated by centrifugation. Analysis of free and polysomal mRNA-protein (mRNP) complexes following ultraviolet-induced cross-linking showed that the free mRNP population was preferentially enriched in PABP. Results of our studies, therefore, suggest that PABP mRNA translation may be repressed by a unique feed-back mechanism.

Xenopus interspersed RNA families, Ocr and XR, bind DNA-binding proteins

Interspersed RNA makes up two-thirds of cytoplasmic polyadenylated RNA in Xenopus and sea urchin eggs. Although it has no known function, previous work has suggested that at least one family of interspersed RNA, XR, binds Xenopus oocyte proteins, and can influence the rate of translation. We have used two Xenopus repeat families, Ocr and XR, to explore their protein binding abilities. Ocr RNA binds the same pattern of highly abundant oocyte proteins that XR RNA binds, which are believed to be messenger ribonucleoprotein (mRNP) particle proteins. In addition, we show that Ocr RNA binds the Oct-60 protein, a member of the POU-domain family of transcription factors found in Xenopus oocytes. Using a 32 base pair sequence from the XR repeat in a DNA affinity column two proteins were isolated, 66 kDa and 92 kDa, that together form a complex with XR DNA. One of these proteins (92 kDa) also binds XR RNA. We suggest that the role of at least a subset of interspersed RNAs in development may be to bind, and sequester in the cytoplasm, DNA-binding proteins until the end of oogenesis.

Multiple sequence elements and a maternal mRNA product control cdk2 RNA polyadenylation and translation during early Xenopus development

Cytoplasmic poly(A) elongation is one mechanism that regulates translational recruitment of maternal mRNA in early development. In Xenopus laevis, poly(A) elongation is controlled by two cis elements in the 3' untranslated regions of responsive mRNAs: the hexanucleotide AAUAAA and a U-rich structure with the general sequence UUUUUAAU, which is referred to as the cytoplasmic polyadenylation element (CPE). B4 RNA, which contains these sequences, is polyadenylated during oocyte maturation and maintains a poly(A) tail in early embryos. However, cdk2 RNA, which also contains these sequences, is polyadenylated during maturation but deadenylated after fertilization. This suggests that cis-acting elements in cdk2 RNA signal the removal of the poly(A) tail at this time. By using poly(A) RNA-injected eggs, we showed that two elements which reside 5' of the CPE and 3' of the hexanucleotide act synergistically to promote embryonic deadenylation of this RNA. When an identical RNA lacking a poly(A) tail was injected, these sequences also prevented poly(A) addition. When fused to CAT RNA, the cdk2 3' untranslated region, which contains these elements, as well as the CPE and the hexanucleotide, promoted poly(A) addition and enhanced chloramphenicol acetyltransferase activity during maturation, as well as repression of these events after fertilization. Incubation of fertilized eggs with cycloheximide prevented the embryonic inhibition of cdk2 RNA polyadenylation but did not affect the robust polyadenylation of B4 RNA. This suggests that a maternal mRNA, whose translation occurs only after fertilization, is necessary for the cdk2 deadenylation or inhibition of RNA polyadenylation. This was further suggested when poly(A)+ RNA isolated from two-cell embryos was injected into oocytes that were then allowed to mature. Such oocytes became deficient for cdk2 RNA polyadenylation but remained proficient for B4 RNA polyadenylation. These data show that CPE function is developmentally regulated by multiple sequences and factors.

Multiple sequence elements and a maternal mRNA product control cdk2 RNA polyadenylation and translation during early Xenopus development

Cytoplasmic poly(A) elongation is one mechanism that regulates translational recruitment of maternal mRNA in early development. In Xenopus laevis, poly(A) elongation is controlled by two cis elements in the 3' untranslated regions of responsive mRNAs: the hexanucleotide AAUAAA and a U-rich structure with the general sequence UUUUUAAU, which is referred to as the cytoplasmic polyadenylation element (CPE). B4 RNA, which contains these sequences, is polyadenylated during oocyte maturation and maintains a poly(A) tail in early embryos. However, cdk2 RNA, which also contains these sequences, is polyadenylated during maturation but deadenylated after fertilization. This suggests that cis-acting elements in cdk2 RNA signal the removal of the poly(A) tail at this time. By using poly(A) RNA-injected eggs, we showed that two elements which reside 5' of the CPE and 3' of the hexanucleotide act synergistically to promote embryonic deadenylation of this RNA. When an identical RNA lacking a poly(A) tail was injected, these sequences also prevented poly(A) addition. When fused to CAT RNA, the cdk2 3' untranslated region, which contains these elements, as well as the CPE and the hexanucleotide, promoted poly(A) addition and enhanced chloramphenicol acetyltransferase activity during maturation, as well as repression of these events after fertilization. Incubation of fertilized eggs with cycloheximide prevented the embryonic inhibition of cdk2 RNA polyadenylation but did not affect the robust polyadenylation of B4 RNA. This suggests that a maternal mRNA, whose translation occurs only after fertilization, is necessary for the cdk2 deadenylation or inhibition of RNA polyadenylation. This was further suggested when poly(A)+ RNA isolated from two-cell embryos was injected into oocytes that were then allowed to mature. Such oocytes became deficient for cdk2 RNA polyadenylation but remained proficient for B4 RNA polyadenylation. These data show that CPE function is developmentally regulated by multiple sequences and factors.

Storage of messenger RNA in eukaryotes: envelopment with protein, translational barrier at 5' side, or conformational masking by 3' side?

Messenger RNA can be stored in the cytoplasm of higher Eukaryotes in the form of masked messenger ribonucleoprotein particles (masked mRNPs, or informosomes). The typical example is the storage of mRNPs in germ cells (oocytes and spermatocytes). The masked mRNPs are inactive in translation, stable, i.e., protected against degradation, and unavailable for poly(A) tail processing, such as cytoplasmic polyadenylation and deadenylation. The major nonspecific mRNA-binding protein forming mRNPs and belonging to a special p50 family of basic, glycine-rich, phosphorylatable proteins seems to be necessary, but not sufficient for the masking. In some cases, mRNA-specific repressor proteins bound to the 5'-untranslated regions (5'-UTR) of mRNAs may be involved. Interactions of the 3'-untranslated regions (3'-UTR) with sequence-specific proteins seem to be of decisive importance for the masking of mRNPs. The hypothesis is proposed that the masking is achieved through a 3'-UTR-induced conformational rearrangement of mRNP; closing into a circle and condensation of mRNP are considered plausible.

Measurement of gut hormone gene expression: mRNA and peptides

During the past decade numerous methods for measurement of mRNA and peptides have been developed. Since the expression cascade from DNA to protein is regulated at all levels, the methods should be carefully designed to accomplish the purpose of the analysis. Regulation of the nuclear processing, the translational activity and the decay of a particular mRNA changes the proportionality between transcriptional activity and production of prepropeptide. Moreover, the post-translational maturation of the pro-hormones may be attenuated. Detection of mRNA is valuable and feasible because it is easy to generate cDNA probes for most hormones, and because mRNA demonstration unequivocally indicates the cellular site of gene expression. The deduction of preprohormone structures has also made it possible to improve the versatility of radioimmunoassays (RIA). Monospecific antibodies and pure tracers have allowed the development of sequence-specific RIA libraries for bioactive peptides and their precursors. Recently we have introduced a simple processing-independent analysis (PIA) for clinical use, since the post-translational maturation of gut peptides may be changed in gastrointestinal diseases. So far PIA has improved the diagnostic sensitivity for gut hormone tumours.

Binding of a phosphoprotein to the 3' untranslated region of the mouse protamine 2 mRNA temporally represses its translation

The synthesis of the protamines, the predominant nuclear proteins of mammalian spermatozoa, is regulated during germ cell development by mRNA storage for about 7 days in the cytoplasm of differentiating spermatids. Two highly conserved sequences, the Y and H elements present in the 3' untranslated regions (UTRs) of all known mammalian protamine mRNAs, form RNA-protein complexes and specifically bind a protein of 18 kDa. Here, we show that translation of fusion mRNAs was markedly repressed in reticulocyte lysates supplemented with a mouse testis extract enriched for the 18-kDa protein when the mRNAs contained the 3' UTR of mouse protamine 2 (mP2) or the Y and H elements of mP2. No significant decrease was seen when the fusion mRNAs contained the 3' UTR of human growth hormone. The 18-kDa protein is developmentally regulated in male germ cells, requires phosphorylation for RNA binding, and is found in the ribonucleoprotein particle fractions of a testicular postmitochondrial supernatant. We propose that a phosphorylated 18-kDa protein plays a primary role in repressing translation of mP2 mRNA by interaction with the highly conserved Y and H elements. At a later stage of male gamete differentiation, the 18-kDa protein no longer binds to the mRNA, likely as a result of dephosphorylation, enabling the protamine mRNA to be translated.

Binding of a phosphoprotein to the 3' untranslated region of the mouse protamine 2 mRNA temporally represses its translation

The synthesis of the protamines, the predominant nuclear proteins of mammalian spermatozoa, is regulated during germ cell development by mRNA storage for about 7 days in the cytoplasm of differentiating spermatids. Two highly conserved sequences, the Y and H elements present in the 3' untranslated regions (UTRs) of all known mammalian protamine mRNAs, form RNA-protein complexes and specifically bind a protein of 18 kDa. Here, we show that translation of fusion mRNAs was markedly repressed in reticulocyte lysates supplemented with a mouse testis extract enriched for the 18-kDa protein when the mRNAs contained the 3' UTR of mouse protamine 2 (mP2) or the Y and H elements of mP2. No significant decrease was seen when the fusion mRNAs contained the 3' UTR of human growth hormone. The 18-kDa protein is developmentally regulated in male germ cells, requires phosphorylation for RNA binding, and is found in the ribonucleoprotein particle fractions of a testicular postmitochondrial supernatant. We propose that a phosphorylated 18-kDa protein plays a primary role in repressing translation of mP2 mRNA by interaction with the highly conserved Y and H elements. At a later stage of male gamete differentiation, the 18-kDa protein no longer binds to the mRNA, likely as a result of dephosphorylation, enabling the protamine mRNA to be translated.

Masking mRNA from translation in somatic cells

We present evidence for a role for Y-box proteins in both mRNA transcription and translation. We have expressed an oocyte-specific RNA-binding protein (FRGY2) in somatic cells. This protein is normally found together with mRNA in large amounts within the masked maternal mRNA storage particles of Xenopus laevis oocytes. Masked maternal mRNA is believed to be translationally repressed during oogenesis and is stored within ribonucleoprotein storage particles for subsequent use during embryogenesis. Surprisingly, FRGY2 is not only found to direct the accumulation of mRNA from promoters containing binding sites for the protein but also to selectively repress translation of the same mRNA. Thus, the role of FRGY2 in the transcription and storage of maternal mRNA is similar to that of the transcription factor TFIIIA in the transcription and storage of 5S rRNA from the oocyte type 5S rRNA genes.

Brain non-adenylated mRNAs

Most eukaryotic messenger RNA (mRNA) species contain a 3'-poly(A) tract. The histone mRNAs are a notable exception although a subclass of histone-encoding mRNAs is polyadenylated. A class of mRNAs lacking a poly(A) tail would be expected to be less stable than poly(A)+ mRNAs and might, like the histones, have a half-life that varied in response to changes in the intracellular milieu. Brain mRNA exhibits an unusually high degree of sequence complexity; studies published ten years ago suggested that a large component of this complexity might be present in a poly(A)- mRNA population that was expressed postnatally. The question of the existence of a complex class of poly(A)- brain mRNAs is particularly tantalizing in light of the heterogeneity of brain cells and the possibility that the stability of these poly(A)- mRNAs might vary with changes in synaptic function, changing hormonal stimulation or with other modulations of neuronal function. The mRNA complexity analyses, although intriguing, did not prove the existence of the complex class of poly(A)- brain mRNAs. The observed mRNA complexity could have resulted from a variety of artifacts, discussed in more detail below. Several attempts have been made to clone members of this class of mRNA. This search for specific poly(A)- brain mRNAs has met with only limited success. Changes in mRNA polyadenylation state do occur in brain in response to specific physiologic stimuli; however, both the role of polyadenylation and de-adenylation in specific neuronal activities and the existence and significance of poly(A)- mRNAs in brain remain unclear.

5' untranslated sequences modulate rapid mRNA degradation mediated by 3' AU-rich element in v-/c-fos recombinants

One major determinant of rapid mRNA decay is the presence of AU-rich sequences located in 3' untranslated regions (UTR). To assess for the contribution of upstream sequences on the activity of the 3' AU-rich destabilizing element, we have determined the decay-rates of v-/c-fos hybrid transcripts by quantitative RNA protection analysis. In a transient expression assay, v-/c-fos recombinants generated two mRNA populations via alternative splicing and removal of an optional intron entirely located in the 5' UTR. Both mRNA species were found to be relatively stable in constructs lacking the c-fos AU-rich destabilizing element. Unexpectedly, in recombinants where intrinsic AU sequences were kept intact, only the full-length mRNA population showed high instability whereas the spliced mRNA species remained relatively stable. A v-/c-fos 5' UTR fragment encoding the optional intron was inserted into alpha-globin genes harboring either the c-fos or GM-CSF destabilizing element. The splicing and degradation patterns of these heterologous transcripts paralleled that of v-/c-fos recombinants. These observations unmasked a 5' cis-acting element in v-/c-fos mRNA whose presence is required for the activity of the AU-rich destabilizing element. They demonstrate the important role of interactions between distinct sequences on the regulation of mRNA stability.

Sequence analysis of cytoplasmic mRNA-binding proteins of Xenopus oocytes identifies a family of RNA-binding proteins

Storage of maternal mRNAs as nontranslated ribonucleoprotein (RNP) complexes is an adaptive strategy in various vertebrate and invertebrate oocytes, for rapid translational recruitment during embryonic development. Previously, we showed that Xenopus laevis oocytes have a soluble cytoplasmic pool of mRNA-binding proteins and particles competent for messenger RNP assembly in vitro. Here we report the isolation of cDNAs for the most abundant messenger RNPs, the 54- and 56-kDa polypeptide (p54/p56) components of the approximately 6S mRNA-binding particle, from an ovarian expression library. The nucleotide sequence of p56 cDNA is almost identical to that recently reported for the putative Xenopus transcription factor FRG Y2. p54 and p56 are highly homologous and are smaller than expected by SDS/PAGE (36 kDa and 37 kDa) due to anomalous electrophoretic mobility. They lack the "RNP consensus motif" but contain four arginine-rich "basic/aromatic islands" that are similar to the RNA-binding domain of bacteriophage mRNA antiterminator proteins and of tat protein of human immunodeficiency virus. The basic/aromatic regions and a second conspicuous 100-amino acid "domain C" of p54 and p56 are conserved in the following DNA-binding proteins: human proteins dpbA, dpbB, and YB-1, rat protein EFIA, and Xenopus protein FRG Y1, all reported to bind to DNA; domain C is homologous to the major Escherichia coli cold-stress-response protein reportedly involved in translational control. Antibodies raised against a peptide of domain C have identified similar proteins in Xenopus somatic cells and in some mammalian cells and tissues. We conclude that p54 and p56 define a family of RNA-binding proteins, at least some of which may be involved in translational regulation.

A 60-kDa protein from rabbit reticulocytes specifically recognizes the capped 5' end of beta-globin mRNA

The binding of proteins from rabbit reticulocyte lysate to in-vitro-generated beta-globin mRNA and its defined segments was investigated using ultraviolet-cross-linking experiments as well as gel-retardation assays. Under stringent conditions, only three proteins (72, 60 and 50 kDa) were found associated with full-length beta-globin mRNA at different positions. The 72-kDa protein is most likely the poly(A)-binding protein and binds, as expected, to the poly(A) tail, whereas the 50-kDa protein exhibits affinity for the trailer region of beta-globin mRNA. The binding region of the 60-kDa protein is located at the 5' end of beta-globin mRNA. The interaction of this protein is dependent on the presence of the 5' cap structure, as indicated by competition experiments using an uncapped beta-globin-mRNA leader segment. Further competition experiments with beta-globin mRNA, deleted in part in the leader region, suggest that, besides the cap structure, certain sequence elements are necessary for the interaction of the 60-kDa protein and the beta-globin mRNA leader.

Cytoplasmic protein binding to highly conserved sequences in the 3' untranslated region of mouse protamine 2 mRNA, a translationally regulated transcript of male germ cells

The expression of the protamines, the predominant nuclear proteins of mammalian spermatozoa, is regulated translationally during male germ-cell development. The 3' untranslated region (UTR) of protamine 1 mRNA has been reported to control its time of translation. To understand the mechanisms controlling translation of the protamine mRNAs, we have sought to identify cis elements of the 3' UTR of protamine 2 mRNA that are recognized by cytoplasmic factors. From gel retardation assays, two sequence elements are shown to form specific RNA-protein complexes. Protein binding sites of the two complexes were determined by RNase T1 mapping, by blocking the putative binding sites with antisense oligonucleotides, and by competition assays. The sequences of these elements, located between nucleotides + 537 and + 572 in protamine 2 mRNA, are highly conserved among postmeiotic translationally regulated nuclear proteins of the mammalian testis. Two closely linked protein binding sites were detected. UV-crosslinking studies revealed that a protein of about 18 kDa binds to one of the conserved sequences. These data demonstrate specific protein binding to a highly conserved 3' UTR of translationally regulated testicular mRNA.

Binding of globin mRNA, beta-globin mRNA segments and RNA homopolymers by immobilized protein of polysomal globin messenger ribonucleoprotein

The binding of rabbit globin mRNA, in-vitro-generated beta-globin mRNA segments, and RNA homopolymers by proteins of rabbit reticulocyte polysomal messenger ribonucleoproteins (mRNP) after SDS gel electrophoresis and electroblotting was examined. The polysomal mRNP proteins have a higher affinity for mRNA than for rRNA and tRNA while having a higher affinity for polypurine than polypyrimidine homopolymers. Binding experiments with synthetic poly(A) and with segments of beta-globin mRNA transcribed from a cDNA in vitro revealed a set of polysomal mRNP proteins which preferentially bind the poly(A)-free beta-globin mRNA. A protein of Mr 90,000 binds specifically the 3'-nontranslated trailer of the poly(A)-free beta-globin mRNA and not the poly(A)-containing globin mRNA. Another set of proteins preferentially binds poly(A). The latter group of proteins contains a prominent species of Mr 72,000, which is most likely the rabbit poly(A)-binding protein. Three polysomal mRNP proteins which bound rabbit globin mRNA did not bind preferentially any of the other RNA probes used.

A specific mRNA binding factor regulates the iron-dependent stability of cytoplasmic transferrin receptor mRNA

Iron regulates human transferrin receptor (hTR) expression by modulating the stability of cytoplasmic hTR mRNA. This regulation requires a distinct secondary structure in the mRNA 3' untranslated region. We identified a specific cytoplasmic factor that binds simultaneously to four homologous palindromes within the regulatory domain. Iron chelator induced the RNA binding activity 25-fold in parallel with mRNA. Upon the addition of iron salts, a rapid decay of factor activity closely preceded hTR mRNA degradation, indicating a causal relation. Induction and decay occurred posttranscriptionally. Binding of the factor to hTR mRNA palindromes was competed by 5' regulatory sequences of ferritin mRNA, which are responsible for iron-dependent translational control. These results suggest that cellular iron maintains its homeostasis by coordinate regulation of hTR and ferritin expression via a common factor.