Alternative poly(A) site utilization during adenovirus infection coincides with a decrease in the activity of a poly(A) site processing factor

The mRNA encoding tauCstF-64 is expressed ubiquitously in mouse tissues

Polyadenylation is a process of endonucleolytic cleavage of the mRNA, followed by addition of up to 250 adenosine residues to the 3' end of the mRNA. Polyadenylation is essential for eukaryotic mRNA expression, and CstF-64 is a subunit of the CstF polyadenylation factor that is required for accurate polyadenylation. We discovered that there are two forms of the CstF-64 protein in mammalian male germ cells, one of which (CstF-64) is expressed in all tissues, the other of which (tauCstF-64) is expressed only in male germ cells and in brain (albeit at significantly lower levels in the brain). Therefore, we were surprised to find that, using reverse transcription-PCR, cDNA cloning, and RNA blot analyses, tauCstF-64 mRNA was expressed at higher levels in brain than in testis. Also, tauCstF-64 mRNA was expressed at lower but detectable levels in all tissues tested, including epididymis, heart, kidney, liver, lung, muscle, ovary, spleen, thymus, and uterus. These results suggest the hypothesis that tauCstF-64 mRNA is regulated at the translational or post-translational level.

Characterization of an upstream regulatory element of adenovirus L1 poly (A) site

The transition from early to late stage infection by adenovirus involves a change in mRNA expression from the adenovirus major late transcription unit (AdMLTU). This early to late switch centers around alternative selection of one of five poly (A) sites (L1-L5) that code for the major structural proteins of Adenovirus. During the early stage of infection, steady state mRNA is primarily derived from the L1 poly (A) site. During the late stage of infection, each of the MLTU poly (A) sites is represented in the steady state mRNA pool (Falck-Pedersen, E., Logan, J., 1989. Regulation of poly(A) site selection in adenovirus. J. Virol. 63 (2), 532-541.). Using transient transfection of a plasmid expressing Chloramphenicol Acetyl Transferase with a tandem poly (A) minigene system (L13) (DeZazzo, J.D., Falck-Pedersen, E., Imperiale, M.J., 1991. Sequences regulating temporal poly(A) site switching in the adenovirus major late transcription unit. Mol. Cell. Biol. 11 (12), 5977-5984; Prescott, J., Falck-Pedersen, E., 1994. Sequence elements upstream of the 3' cleavage site confer substrate strength to the adenovirus L1 and L3 polyadenylation sites. Mol. Cell. Biol. 14 (7), 4682-4693.), it has been demonstrated that the promoter-proximal L1 poly (A) site which is poorly recognized by the 3' end processing machinery, contains an upstream repressor element (URE) that influences steady state levels of mRNA (Prescott, J.C., Liu, L., Falck-Pedersen, E., 1997. Sequence-mediated regulation of adenovirus gene expression by repression of mRNA accumulation. Mol. Cell. Biol. 17 (4), 2207-2216.). In this study, we have further characterized the elements that mediate L1URE function. These studies indicate that the L1 upstream regulatory element (L1 URE) contains a complex RNA architecture that serves to repress gene expression through multiple sub-effectors. The L1URE functions when located upstream of a heterologous poly (A) site, and is able to strongly suppress steady state mRNA expression from the MLTU L3 poly (A) site or the murine beta-globin poly (A) site. In the tandem L13 mini-gene system, the L1URE is revealed to influence steady state mRNA stability, and subdomains within L1URE influence both gene expression and the steady state ratio of transcripts processed at proximal versus distal poly (A) sites. Transcript and gene repression mediated by the L1URE may provide a general strategy employed by adenovirus to block premature expression of late stage MLTU transcripts.

A history of poly A sequences: from formation to factors to function

Biological polyadenylation, first recognized as an enzymatic activity, remained an orphan enzyme until poly A sequences were found on the 3' ends of eukarvotic mRNAs. Their presence in bacteria viruses and later in archeae (ref. 338) established their universality. The lack of compelling evidence for a specific function limited attention to their cellular formation. Eventually the newer techniques of molecular biology and development of accurate nuclear processing extracts showed 3' end formation to be a two-step process. Pre-mRNA was first cleaved endonucleolytically at a specific site that was followed by sequential addition of AMPs from ATP to the 3' hydroxyl group at the end of mRNA. The site of cleavage was specified by a conserved hexanucleotide, AAUAAA, from 10 to 30 nt upstream of this 3' end. Extensive purification of these two activities showed that more than 10 polypeptides were needed for mRNA 3' end formation. Most of these were in complexes involved in the cleavage step. Two of the best characterized are CstF and CPSF, while two other remain partially purified but essential. Oddly, the specific proteins involved in phosphodiester bond hydrolysis have yet to be identified. The polyadenylation step occurs within the complex of poly A polymerase and poly A-binding protein, PABII, that controls poly A length. That the cleavage complex, CPSF, is also required for this step attests to a tight coupling of the two steps of 3' and formation. The reaction reconstituted from these RNA-free purified factors correctly processes pre-mRNAs. Meaningful analysis of the role of poly A in mRNA metabolism or function was possible once quantities of these proteins most often over-expressed from cDNA clones became available. The large number needed for two simple reactions of an endonuclease, a polymerase and a sequence recognition factor, pointed to 3' end formation as a regulated process. Polyadenylation itself had appeared to require regulation in cases where two poly A sites were alternatively processed to produce mRNA coding for two different proteins. The 64-KDa subunit of CstF is now known to be a regulator of poly A site choice between two sites in the immunoglobulin heavy chain of B cells. In resting cells the site used favors the mRNA for a membrane-bound protein. Upon differentiation to plasma cells, an upstream site is used the produce a secreted form of the heavy chain. Poly A site choice in the calcitonin pre-mRNA involves splicing factors at a pseudo splice site in an intron downstream of the active poly site that interacts with cleavage factors for most tissues. The molecular basis for choice of the alternate site in neuronal tissue is unknown. Proteins needed for mRNA 3' end formation also participate in other RNA-processing reactions: cleavage factors bind to the C-terminal domain of RNA polymerase during transcription; splicing of 3' terminal exons is stimulated port of by cleavage factors that bind to splicing factors at 3' splice sites. nuclear ex mRNAs is linked to cleavage factors and requires the poly A II-binding protein. Most striking is the long-sought evidence for a role for poly A in translation in yeast where it provides the surface on which the poly A-binding protein assembles the factors needed for the initiation of translation. This adaptability of eukaryotic cells to use a sequence of low information content extends to bacteria where poly A serves as a site for assembly of an mRNA degradation complex in E. coli. Vaccinia virus creates mRNA poly A tails by a streamlined mechanism independent of cleavage that requires only two proteins that recognize unique poly A signals. Thus, in spite of 40 years of study of poly A sequences, this growing multiplicity of uses and even mechanisms of formation seem destined to continue.

Reexamining the polyadenylation signal: were we wrong about AAUAAA?

Polyadenylation is the process by which most eukaryotic mRNAs form their 3' ends. It was long held that polyadenylation required the sequence AAUAAA and that 90% of mRNAs had AAUAAA within 30 nucleotides of the site of poly(A) addition. More recent studies, aided by computer analysis of sequences made available in GenBank and expressed sequence tag (EST) databases, have suggested that the actual incidence of AAUAAA is much lower, perhaps as low as 50-60%. Reproductive biologists have long recognized that a large number of mRNAs in male germ cells of mammals lack AAUAAA but are otherwise normally polyadenylated. Recent research in our laboratory has uncovered a new form of an essential polyadenylation protein, tauCstF-64, that is most highly expressed in male germ cells, and to a smaller extent in the brain, and which we propose plays a significant role in AAUAAA-independent mRNA polyadenylation in germ cells.

RNA polymerase II-dependent positional effects on mRNA 3' end processing in the adenovirus major late transcription unit

During the early phase of adenovirus infection, the promoter-proximal L1 poly(A) site in the major late transcription unit is used preferentially despite the fact that the distal L3 poly(A) site is stronger (i.e. it competes better for processing factors and is cleaved at a faster rate, in vitro). Previous work had established that this was due at least in part to the stable binding of the processing factor, cleavage and polyadenylation specificity factor, to the L1 poly(A) site as mediated by specific regulatory sequences. It is now demonstrated that in addition, the L1 poly(A) site has a positional advantage because of its 5' location in the transcription unit. We also show that preferential processing of a particular poly(A) site in a complex transcription unit is dependent on RNA polymerase II. Our results are consistent with recent reports demonstrating that the processing factors cleavage and polyadenylation specificity factor and cleavage stimulatory factor are associated with the RNA polymerase II holoenzyme; thus, processing at a weak poly(A) site like L1 can be enhanced by virtue of its being the first site to be transcribed.

Elevated levels of the polyadenylation factor CstF 64 enhance formation of the 1kB Testis brain RNA-binding protein (TB-RBP) mRNA in male germ cells

The single copy mouse Testis Brain RNA-Binding Protein (TB-RBP) gene encodes three mRNAs of 3.0, 1.7, and 1.0 kb which only differ in their 3' UTRs. The 1 kb TB-RBP mRNA predominates in testis, while somatic cells preferentially express the 3.0 kb TB-RBP mRNA. Here we show that the 1 kb mRNA is translated several-fold more efficiently than the 3 kb TB-RBP in rabbit reticulocyte lysates and cells with elevated levels of the 1 kB TB-RBP mRNA express high levels of TB-RBP. To determine if the cleavage stimulatory factor CstF 64 can modulate the alternative splicing of the TB-RBP pre-mRNA and therefore TB-RBP expression, CstF 64 levels and binding to alternative polyadenylation sites were examined. CstF 64 is abundant in the testis and preferentially binds to a distal site in the TB-RBP pre-mRNA that produces the 3 kb TB-RBP. Moreover, upregulation or overexpression of CstF 64 increases the poly(A) site selection for the 1 kb TB-RBP mRNA. We propose that the level of the polyadenylation factor CstF 64 modulates the level of TB-RBP synthesis in male germ cells by an alternative processing of the TB-RBP pre-mRNA.

Tissue-specific autoregulation of Drosophila suppressor of forked by alternative poly(A) site utilization leads to accumulation of the suppressor of forked protein in mitotically active cells

The Suppressor of forked protein is the Drosophila homolog of the 77K subunit of human cleavage stimulation factor, a complex required for the first step of the mRNA 3'-end-processing reaction. We have shown previously that wild-type su(f) function is required for the accumulation of a truncated su(f) transcript polyadenylated in intron 4 of the gene. This led us to propose a model in which the Su(f) protein would negatively regulate its own accumulation by stimulating 3'-end formation of this truncated su(f) RNA. In this article, we demonstrate this model and show that su(f) autoregulation is tissue specific. The Su(f) protein accumulates at a high level in dividing tissues, but not in nondividing tissues. We show that this distribution of the Su(f) protein results from stimulation by Su(f) of the tissue-specific utilization of the su(f) intronic poly(A) site, leading to the accumulation of the truncated su(f) transcript in nondividing tissues. Utilization of this intronic poly(A) site is affected in a su(f) mutant and restored in the mutant with a transgene encoding wild-type Su(f) protein. These data provide an in vivo example of cell-type-specific regulation of a protein level by poly(A) site choice, and confirm the role of Su(f) in regulation of poly(A) site utilization.

CD164--a novel sialomucin on CD34+ cells

Hematopoiesis in adult bone marrow is a tightly regulated process involving interactions between cytokine and adhesion receptors on hematopoietic progenitor cells and their cognate ligands in the immediate microenvironment. These interactions control hematopoietic stem cell self-renewal, quiescence, commitment and migration. Recently, sialomucins have assumed some importance in hematopoiesis, with six of these receptors, CD34, PSGL-1, CD43, PCLP, CD45RA and CD164, having been identified on primitive hematopoietic precursor cells and/or their associated stromal/endothelial elements. This article reviews the cloning, expression and function of the recently identified sialomucin, CD164, which is highly expressed by primitive hematopoietic progenitor cells. The CD164 receptor is implicated in mediating or regulating hematopoietic precursor cell adhesion to stroma, and may serve as a potent negative regulator of hematopoietic progenitor cell proliferation.

Formation of mRNA 3' ends in eukaryotes: mechanism, regulation, and interrelationships with other steps in mRNA synthesis

Formation of mRNA 3' ends in eukaryotes requires the interaction of transacting factors with cis-acting signal elements on the RNA precursor by two distinct mechanisms, one for the cleavage of most replication-dependent histone transcripts and the other for cleavage and polyadenylation of the majority of eukaryotic mRNAs. Most of the basic factors have now been identified, as well as some of the key protein-protein and RNA-protein interactions. This processing can be regulated by changing the levels or activity of basic factors or by using activators and repressors, many of which are components of the splicing machinery. These regulatory mechanisms act during differentiation, progression through the cell cycle, or viral infections. Recent findings suggest that the association of cleavage/polyadenylation factors with the transcriptional complex via the carboxyl-terminal domain of the RNA polymerase II (Pol II) large subunit is the means by which the cell restricts polyadenylation to Pol II transcripts. The processing of 3' ends is also important for transcription termination downstream of cleavage sites and for assembly of an export-competent mRNA. The progress of the last few years points to a remarkable coordination and cooperativity in the steps leading to the appearance of translatable mRNA in the cytoplasm.

Regulation of poly(A) site choice of several yeast mRNAs

Several yeast genes produce multiple transcripts with different 3'-ends. Of these, four genes are known to produce truncated transcripts that end within the coding sequence of longer transcripts: CBP1 , AEP2 / ATP13 , RNA14 and SIR1 . It has been shown that the level of the truncated CBP1 transcript increases during the switch to respiratory growth while that of the full-length transcript decreases. To determine whether this phenomenon is unique to CBP1 , northern analysis was used to determine whether the levels of other truncated transcripts are regulated similarly by carbon source. The levels of the shortest transcripts of AEP2 / ATP13 and RNA14 increased during respiration while the shortest SIR1 transcript remained constant. However, two longer SIR1 transcripts were regulated reciprocally by carbon source. Mapping the 3'-ends of each transcript by sequencing partial cDNA clones revealed multiple 3'-ends for each transcript. Examination of the sequences surrounding the 3'-ends of the induced transcripts failed to identify a consensus sequence but did reveal weak putative 3'-end formation signals in all of the transcripts. Similarly, no consensus sequence was found when the sequences surrounding the 3'-ends of the longest transcripts were compared, but again weak putative 3'-end formation signals were identified. These data are suggestive of carbon source regulation of alternative poly(A) site choice in yeast.

Influenza virus NS1 protein interacts with the cellular 30 kDa subunit of CPSF and inhibits 3'end formation of cellular pre-mRNAs

Inhibition of the nuclear export of poly(A)-containing mRNAs caused by the influenza A virus NS1 protein requires its effector domain. Here, we demonstrate that the NS1 effector domain functionally interacts with the cellular 30 kDa subunit of CPSF, an essential component of the 3' end processing machinery of cellular pre-mRNAs. In influenza virus-infected cells, the NS1 protein is physically associated with CPSF 30 kDa. Binding of the NS1 protein to the 30 kDa protein in vitro prevents CPSF binding to the RNA substrate and inhibits 3' end cleavage and polyadenylation of host pre-mRNAs. The NS1 protein also inhibits 3' end processing in vivo, and the uncleaved pre-mRNA remains in the nucleus. Via this novel regulation of pre-mRNA 3' end processing, the NS1 protein selectively inhibits the nuclear export of cellular, and not viral, mRNAs.

The suppressor of forked protein of Drosophila, a homologue of the human 77K protein required for mRNA 3'-end formation, accumulates in mitotically-active cells

The suppressor of forked (Su(f)) protein of Drosophila melanogaster is highly homologous to two proteins involved in mRNA 3'-end formation, the yeast RNA14 protein and the 77K subunit of human cleavage stimulation factor (CstF). This suggests a role for su(f) in mRNA 3'-end-processing, probably as part of Drosophila CstF. We have investigated the expression pattern of su(f) during Drosophila development and found that the su(f) gene product is not detected ubiquitously. The Su(f) protein accumulates in mitotically-active cells, but does not in non-dividing cells. This expression pattern corroborates earlier data suggesting that the phenotypes of su(f) mutants could result from a defect in cell proliferation. Our results suggest that, in Drosophila, Su(f) is involved in the regulatory function of CstF.

RNA ligands selected by cleavage stimulation factor contain distinct sequence motifs that function as downstream elements in 3'-end processing of pre-mRNA

Critical events in 3'-end processing of pre-mRNA are the recognition of the AAUAAA polyadenylation signal by cleavage and polyadenylation specificity factor (CPSF) and the binding of cleavage stimulation factor (CstF) via its 64-kDa subunit to the downstream element. The stability of this CPSF.CstF.RNA complex is thought to determine the efficiency of 3'-end processing. Since downstream elements reveal high sequence variability, in vitro selection experiments with highly purified CstF were performed to investigate the sequence requirements for CstF-RNA interaction. CstF was purified from calf thymus and from HeLa cells. Surprisingly, calf thymus CstF contained an additional, novel form of the 64-kDa subunit with a molecular mass of 70 kDa. RNA ligands selected by HeLa and calf thymus CstF contained three highly conserved sequence elements as follows: element 1 (AUGCGUUCCUCGUCC) and two closely related elements, element 2a (YGUGUYN0-4UUYAYUGYGU) and element 2b (UUGYUN0-4AUUUACU(U/G)N0-2YCU). All selected sequences tested functioned as downstream elements in 3'-end processing in vitro. A computer survey of the EMBL data library revealed significant homologies to all selected elements in naturally occurring 3'-untranslated regions. The majority of element 2a homologies was found downstream of coding sequences. Therefore, we postulate that this element represents a novel consensus sequence for downstream elements in 3'-end processing of pre-mRNA.

Molecular cloning and expression of an interferon-inducible protein encoded by gene 203 from the gene 200 cluster

We report here the complete coding sequence of a 203 cDNA, a member of the interferon-inducible Ifi 200 gene family. By combining reverse-transcriptase PCR and rapid amplification of cDNA ends (RACE) techniques we have obtained a 3.8-kb cDNA corresponding to a 203 mRNA. When used as a probe in northern analysis, its 3' segment hybridized to a 3.8-kb interferon-inducible mRNA, whereas the 5'-end additionally hybridized to a less abundant interferon-inducible 1.8-kb mRNA. Nucleotide sequence analysis revealed that the two mRNAs share the 5'-untranslated region and the same open reading frame, which encodes a hydrophilic protein composed of 408 amino acids. The difference between them is due to a 3'-untranslated region extended by alternative polyadenylation site selection. Furthermore, 203 mRNA was found to be inducible by interferon-alpha in various murine cell lines. Using polyclonal antibodies raised against a segment specific for the 203 protein, we established that p203 protein levels increase on treatment with interferon-alpha in murine fibroblasts and that p203 is located in the nucleus.

Expression of the release factor eRF1 (Sup45p) gene of higher eukaryotes in yeast and mammalian tissues

Polypeptide chain termination in eukaryotic cells is mediated in part by the release factor eRF1 (Sup45p). We have isolated and characterised cDNAs encoding this translation factor from Syrian hamster (Mesocricetus auratus) and human (Homo sapiens) Daudi cells. Comparison of the deduced amino acid sequence of these new eRF1 (Sup45p) sequences with those published for Saccharomyces cerevisiae, Arabidopsis thaliana, Xenopus laevis and human indicates a high degree of amino acid identity across a broad evolutionary range of species. Both the 5' and 3' UTRs of the mammalian eRF1 (Sup45p)-encoding cDNAs show an unusually high degree of conservation for non-coding regions. In addition, the presence of two different lengths of 3' UTR sequences in the mammalian eRF1 (Sup45p) cDNAs indicated that alternative polyadenylation sites might be used in vivo. Northern blot analysis demonstrated that eRF1 (Sup45p) transcripts of differing length, consistent with the use of alternative polyadenylation sites, were detectable in a wide range of mammalian tissues. The Xenopus, human and Syrian hamster eRF1 (Sup45p) cDNAs were shown to support the viability of a strain of S cerevisiae carrying an otherwise lethal sup45::HIS3 gene disruption indicating evolutionary conservation of function. However, the yeast strains expressing the heterogenous eRF1 (Sup45p) showed a defect in translation termination as defined by an enhancement of nonsense suppressor tRNA activity in vivo. Western blot analysis confirmed that Xenopus eRF1 (Sup45p) was primarily ribosome-associated when expressed in yeast indicating that the ribosome-binding domain of eRF1 (Sup45p) is also conserved.

Inducible nuclear factors binding the IgM heavy chain pre-mRNA secretory poly(A) site

Two alternative forms of IgM heavy-chain mRNA are produced from a common precursor mRNA as a result of competition between cleavage/poly(A) addition at the upstream (secretory) poly(A) site and cleavage/poly(A) addition at the downstream (membrane) poly(A) site coupled with splicing. The efficiency of cleavage at the secretory poly(A) site is thought to play a crucial role in this alternative processing. We therefore examined RNA binding factors recognizing the secretory poly(A) site, in the absence of the splicing option, to look for transacting factors that may play a role in cleavage/polyadenylation efficiency at this site. Purified primary B cells produce the secretory form of mu mRNA when stimulated with lipopolysaccharide (LPS) and the membrane form of mu mRNA when their antigen receptors are ligated by anti-mu antibodies. We compared RNA binding factors in nuclear extracts from cells produced by these different stimulatory conditions and show that induction of the secretory form of mu mRNA by LPS correlates with the induction of a 28-32-kDa secretory poly(A) site-specific polypeptide which is also present in the plasmacytoma cell line J558L. Visualization of the 28-32-kDa polypeptide in UV cross-linking assays depends on a GU-rich element downstream of the secretory poly(A) site. We show that this GU-rich region enhances polyadenylation efficiency in vivo by transfection of luciferase reporter constructs into the plasmacytoma J558L. We also examined nuclear extracts from B cells doubly stimulated with LPS and anti-mu antibodies in which expression of the secretory form of mu mRNA is selectively inhibited. This inhibition may be due to a down-regulation of polyadenylation at the secretory poly(A) site or an up-regulation of the competitive splicing process. This form of stimulation does not lead to the disappearance of the 28-32-kDa polypeptide, but to an enhanced binding of a 50-55-kDa factor which binds both the secretory and membrane poly(A) site. We report the first detection of changes in RNA binding factors taking place at the secretory poly(A) site which correlate with the expression of different forms of mu mRNA produced by primary B cells under different stimulation conditions.

The polyadenylation factor CstF-64 regulates alternative processing of IgM heavy chain pre-mRNA during B cell differentiation

The switch from membrane-bound to secreted-form IgM that occurs during differentiation of B lymphocytes has long been known to involve regulated processing of the heavy chain pre-mRNA. Here, we show that accumulation of one subunit of an essential polyadenylation factor (CstF-64) is specifically repressed in mouse primary B cells and that overexpression of CstF-64 is sufficient to switch heavy chain expression from membrane-bound (microm) to secreted form (micros). We further show that CstF-64 is limiting for formation of intact CstF, that CstF has a higher affinity for the microm poly(A) site than for the micros site, and that the microm site is stronger in a reconstituted in vitro processing reaction. Our results indicate that CstF-64 plays a key role in regulating IgM heavy chain expression during B cell differentiation.

Distinct differences in the requirements for ribonucleoprotein complex formation on differentially regulated pre-edited mRNAs in Trypanosoma brucei

Incubation of synthetic pre-edited mRNAs with extracts of Trypanosoma brucei mitochondria results in a family of specific, stable ribonucleoprotein (RNP) complexes that can be visualized by non-denaturing gel electrophoresis. We compared complexes that form with a substrate corresponding to the ATPase 6 (A6) pre-mRNA 3' region that is edited in both bloodstream and procyclic forms with a substrate corresponding to the 5' end of apocytochrome b (CYb) pre-mRNA that is edited only in procyclic (insect) forms. Four to five complexes are detected with both substrates and are specific since competition with homologous but not heterologous substrates prevents their formation. Formation of the CYb complex, however, is more sensitive to heterologous RNAs. In addition, the A6 substrate is more effective at preventing formation of CYb complexes than the converse. CYb complex formation is also more sensitive to divalent cation and salt concentrations and formation of one A6 component has a temperature optimum of 37 degrees C while that of CYb is 27 degrees C.

Characterization and expression of iron regulatory protein 2 (IRP2). Presence of multiple IRP2 transcripts regulated by intracellular iron levels

Iron regulatory proteins (IRP1 and IRP2) are RNA-binding proteins that bind to stem-loop structures, termed iron-responsive elements (IREs), present in either the 5'- or 3'-untranslated regions of specific mRNAs. The binding of IRPs to 5'-IREs inhibits translation of mRNA, whereas the binding of IRPs to 3'-IREs stabilizes mRNA. To study the structure and regulation of IRP2, we isolated cDNAs for rat and human IRP2. The derived amino acid sequence of rat IPR2 is 93% identical with that of human IRP2 and is present in lower eukaryotes, indicating that IRP2 is highly conserved. IRP1 and IRP2 share 61% overall amino acid identity. IRP2 is ubiquitously expressed in rat tissues, the highest amounts present in skeletal muscle and heart. IRP2 is encoded by multiple mRNAs of 6.4, 4.0, and 3.7 kilobases. The 3'-untranslated region of rat IRP2 contains multiple polyadenylation signals, two of which could account for the 4.0-kb and 3.7-kb mRNAs. The 3.7-kb mRNA is increased in iron-depleted cells and occurs with a reciprocal decrease in the 6.4-kb transcript. These data suggest that the 3.7-kb mRNA is produced by alternative poly(A) site utilization in iron-depleted cells.

Transcription of the rat beta 1-adrenergic receptor gene. Characterization of the transcript and identification of important sequences

We have characterized the 5' and 3' ends of the rat beta 1-adrenergic receptor transcript using RNase protection assays and have used transient transfection analysis to identify regions of the beta 1-adrenergic gene 5'-flanking sequences which are important for expression. The transcript has multiple start sites, occurring primarily in two clusters at bases -250 and -280, relative to the first base of the initiation codon. Two potential polyadenylation signals at +2450 and +2732 are both functional, although the site at +2732 is preferred both in C6 glioma cells and in heart tissue. Characterization of the gene by transient transfection analysis has identified a region between bases -389 and -325 which is necessary for expression. The specific deletion of a potentially functional inverted CCAAT sequence within this region does not significantly alter activity. In addition to the region from -389 and -325, deletion of the bases between -1 and -159 and between -186 and -211 significantly alters expression. Both of these regions are down-stream from the beta 1-adrenergic receptor gene start sites and may function either through regulation of transcription or through alteration of the transcript structure.