Vimentin gene expression during myogenesis: two functional transcripts from a single copy gene

Intermediate filaments in cardiomyopathy

Intermediate filament (IF) proteins are critical regulators in health and disease. The discovery of hundreds of mutations in IF genes and posttranslational modifications has been linked to a plethora of human diseases, including, among others, cardiomyopathies, muscular dystrophies, progeria, blistering diseases of the epidermis, and neurodegenerative diseases. The major IF proteins that have been linked to cardiomyopathies and heart failure are the muscle-specific cytoskeletal IF protein desmin and the nuclear IF protein lamin, as a subgroup of the known desminopathies and laminopathies, respectively. The studies so far, both with healthy and diseased heart, have demonstrated the importance of these IF protein networks in intracellular and intercellular integration of structure and function, mechanotransduction and gene activation, cardiomyocyte differentiation and survival, mitochondrial homeostasis, and regulation of metabolism. The high coordination of all these processes is obviously of great importance for the maintenance of proper, life-lasting, and continuous contraction of this highly organized cardiac striated muscle and consequently a healthy heart. In this review, we will cover most known information on the role of IFs in the above processes and how their deficiency or disruption leads to cardiomyopathy and heart failure.

Increased expression and activity of ornithine decarboxylase in chicks genetically selected for rapid growth rate

Polyamines are essential requirements for cell proliferation and their role in stimulation of RNA, DNA, and protein synthesis is clearly established. Ornithine decarboxylase is a key enzyme in the biosynthesis of the polyamines and its activity is regulated in response to factors that stimulate cell proliferation. Steady-state ODC mRNA levels and enzyme activity were measured in muscle of chicks genetically selected for increased growth rate or for egg production. In muscle, muscle satellite cells and myotubes, two ODC mRNA transcripts are present of molecular size 2.05 and 1.75 kb. Northern blotting analysis suggest that these transcripts are produced as a result of using different polyadenylation sites. Between day 1 and day 6 after hatching, a period of rapid muscle growth in these animals, a peak in muscle ODC mRNA levels is followed by a peak in enzyme activity in both lines. Significantly higher ODC mRNA levels and enzyme activity are associated with selection for rapid growth in the broiler line. The results are consistent with other data showing that ODC is a major factor in cell growth and provide further evidence that it is a candidate 'trait-gene' for growth.

Transient expression of the intermediate filament nestin during skeletal muscle development

It has previously been established that skeletal muscle development is accompanied by changes in the composition of intermediate filaments: vimentin is expressed predominantly in myoblasts and desmin in adult myotubes. We show that the intermediate filament transitions during muscle development are more complex, and involve a transient expression of the recently discovered intermediate filament nestin. Nestin RNA is expressed predominantly early, in a biphasic pattern, and is markedly downregulated in adult rat muscle, whereas desmin RNA becomes more abundant throughout development. Nestin protein was found up to the postnatal myotube stage, where it colocalized with desmin in Z bands. The intracellular distribution of nestin, vimentin and desmin was analysed in the human myogenic cell line G6 before and after in vitro differentiation. Despite its more distant evolutionary and structural relationship to the other two intermediate filaments, nestin formed a cytoplasmic filamentous network indistinguishable from that of desmin and vimentin, both in undifferentiated myoblasts and after differentiation to multinuclear myotubes. In conclusion, our data suggest that nestin is an integrated component of the dynamic intermediate filament network during muscle development and that nestin copolymerizes with desmin and vimentin at stages of coexpression.

Primary structure and lens-specific expression of genes for an intermediate filament protein and a beta-tubulin in cephalopods

Intermediate filament (IF) protein and tubulin cDNAs of cephalopod eye lenses were cloned and sequenced. The rod regions of the deduced IF proteins of the squid and octopus were more similar (68% identical) than were head (33% identical) and tail (40% identical) regions. The rod sequences were closer to squid neuronal IF protein (39% identical) than to any other known IF protein. There was only 31% identity between the rod regions, 21-30% identity between the head regions and 23-32% identity between the tail regions of the present IF proteins of cephalopods and other invertebrates. The rod regions of the cephalopod IF proteins contained the 6 heptads characteristic of nuclear lamins, consistent with an evolutionary relationship between IF proteins and lamins. The present octopus alpha-tubulin was 93% and beta-tubulin was 87% identical to the corresponding tubulins of insects and vertebrates. SDS-PAGE and peptide sequencing indicated that the order of abundance of the cephalopod lens cytoskeletal proteins was IF proteins, actin and tubulins. Northern blot hybridization revealed a 4 kb mRNA for the octopus IF protein and 2.9 and 7.3 kb mRNAs for the squid IF protein; the alpha-tubulin mRNA was about 1.8 kb in the octopus and squid, and the beta-tubulin mRNA was about 2.8 kb in the octopus. The alpha-tubulin mRNA was present in all tissues examined; by contrast, the present beta-tubulin and IF protein mRNAs appeared specialized for lens expression.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Expression of 9E3 mRNA is associated with mitogenicity, phosphorylation, and morphological alteration in chicken embryo fibroblasts

Transformation of chicken embryo fibroblasts (CEF) with viruses encoding src, ros, yes, and fps as well as ras, mos, middle T, erbA and erbB, myc, and crk stimulated 9E3 mRNA expression. Treatment of CEF with agents that modulate cell shape or attachment to the substratum caused an increase in 9E3 mRNA without an increase in tyrosine phosphorylation. 9E3 mRNA was also increased in CEF in response to several agents which modulate phosphorylation, including phorbol myristic acetate, vanadate, and okadaic acid, which suggests that the rapid induction of 9E3 mRNA expression in CEF by the src protein occurs downstream of morphological or phosphorylation events.

Transient expression of an intermediate filament-associated protein (IFAPa-400) during in vivo and in vitro differentiation of chick embryonic cells derived from neuroectoderm

The expression of an intermediate filament (IF) associated protein (IFAPa-400) and IF proteins was investigated during chick neurogenesis. Using immunoblots and indirect immunofluorescence we have found that IFAPa-400 was strongly expressed during the early events of nervous tissue ontogenesis and disappeared thereafter. IFAPa-400 was elevated in the brain and retina until ED 10 and until hatching in the cerebellum. This protein was shown to be transiently expressed in the Müller glia of the developing retina. In the brain, IFAPa-400 decreased as development proceeded in a way similar to vimentin but the latter remained elevated in the retina and the cerebellum radial glia (Müller and Bergmann cells). In all tissues examined, GFAP was detected long after the disappearance of IFAPa-400. In dorsal root ganglion cell cultures, IFAPa-400 and vimentin were absent from mature neurons but were coexpressed into supportive cells. In trunk neural crest cell cultures, IFAPa-400 and vimentin were present in all cells after one day but IFAPa-400 became undetectable after a few days of culture in differentiated melanocytes and catecholaminergic neurons. The transient expression of this giant cytoskeletal protein in non-differentiated cells deriving from the neuroectoderm could reflect a structural change which precedes overt cytodifferentiation.

A negative element involved in vimentin gene expression

Vimentin is one member of the intermediate filament multigene family which exhibits both tissue- and developmental stage-specific expression. In vivo, vimentin is expressed in cells of mesenchymal origin. Previously, we identified both enhancer and promoter elements in the chicken vimentin gene which regulate gene expression in a positive manner. In this report, we have identified a 40-base-pair region at -568 base pairs between the proximal and distal enhancer elements which represses transcriptional activity. This silencer region can also repress the heterologous herpes simplex virus thymidine kinase promoter, which is comparable to the vimentin promoter. In addition, the element is able to function in a position- and orientation-independent manner, and the amount of repression is increased by multiple copies. Here we show by gel retardation assays and DNase I footprinting that this region binds a protein in nuclear extracts from HeLa cells. Southwestern (DNA-protein) blot analysis indicates this protein is approximately 95 kilodaltons in size. Moreover, protein distribution and activity mimic the expression pattern of vimentin during myogenesis, i.e., protein binding increases as vimentin gene expression decreases. The silencer region shares strong sequence similarity with 5'-flanking sequences found in both the human and hamster vimentin genes and with other characterized silencer elements, including the human immunodeficiency virus long terminal repeat, rat growth hormone, chicken lysozyme, and rat insulin genes. Thus, a negative element appears to bind a 95-kilodalton protein involved in regulating the tissue-specific expression of the chicken vimentin gene.

Regulation of vimentin gene expression in the ocular lens

Vimentin expression in the lens is striking due to the reported mesenchymal preference of vimentin and the epithelial origin of the lens. The amount of chicken vimentin mRNA levels determined by Northern blot analysis increased 3-fold from 7 to 14 days of embryonic lens development and then decreased 10-fold at 16 days of development, suggesting that post-transcriptional processes may contribute to the level of cytoplasmic vimentin mRNA during lens development. To analyze the mechanisms governing vimentin gene expression in the lens at the level of transcription, a series of chicken vimentin 5'-flanking region deletions were fused to the bacterial CAT gene and transfected into fibroblasts and lens cultures derived from three species. The -160 to +1 sequence conferred equal promoter activity in cultured chicken lens epithelial cells and fibroblasts. The -321 to -160 sequences increased promoter activity in all cultures, but more strongly in fibroblasts than in lens cells. Sequence elements in the region -608 to -321 repressed promoter activity in lens cells and fibroblasts. Promoter activity was partially restored in fibroblasts but not in lens cells by -767 to -608 sequences. Vimentin gene expression in the lens thus appears to be controlled by multiple positive- and negative-acting elements in its 5'-flanking sequence.

A negative element involved in vimentin gene expression

Vimentin is one member of the intermediate filament multigene family which exhibits both tissue- and developmental stage-specific expression. In vivo, vimentin is expressed in cells of mesenchymal origin. Previously, we identified both enhancer and promoter elements in the chicken vimentin gene which regulate gene expression in a positive manner. In this report, we have identified a 40-base-pair region at -568 base pairs between the proximal and distal enhancer elements which represses transcriptional activity. This silencer region can also repress the heterologous herpes simplex virus thymidine kinase promoter, which is comparable to the vimentin promoter. In addition, the element is able to function in a position- and orientation-independent manner, and the amount of repression is increased by multiple copies. Here we show by gel retardation assays and DNase I footprinting that this region binds a protein in nuclear extracts from HeLa cells. Southwestern (DNA-protein) blot analysis indicates this protein is approximately 95 kilodaltons in size. Moreover, protein distribution and activity mimic the expression pattern of vimentin during myogenesis, i.e., protein binding increases as vimentin gene expression decreases. The silencer region shares strong sequence similarity with 5'-flanking sequences found in both the human and hamster vimentin genes and with other characterized silencer elements, including the human immunodeficiency virus long terminal repeat, rat growth hormone, chicken lysozyme, and rat insulin genes. Thus, a negative element appears to bind a 95-kilodalton protein involved in regulating the tissue-specific expression of the chicken vimentin gene.

Modulation of arachidonic acid metabolism by Rous sarcoma virus

Arachidonic acid (C20:4) metabolites were released constitutively from wild-type Rous sarcoma virus-transformed chicken embryo fibroblasts (CEF). 3H-labeled C20:4 and its metabolites were released from unstimulated and uninfected CEF only in response to stimuli such as serum, phorbol ester, or the calcium ionophore A23187. High-pressure liquid chromatography analysis showed that the radioactivity released from [3H]arachidonate-labeled transformed cells was contained in free arachidonate and in the cyclooxygenase products prostaglandin E2 and prostaglandin F2 alpha; no lipoxygenase products were identified. The release of C20:4 and its metabolites from CEF infected with pp60src deletion mutants was correlated with serum-independent DNA synthesis and with the expression of the mRNA for 9E3, a gene expressed in Rous sarcoma virus-transformed cells which has homology with several mitogenic and inflammatory peptides. 3H-labeled C20:4 release was not correlated with p36 phosphorylation, which argues against a role for this protein as a phospholipase A2 inhibitor. CEF infected with other oncogenic viruses encoding a tyrosine kinase also released C20:4, as did CEF infected with viruses that contained mos and ras; however, infection with a crk-containing virus did not result in stimulation of 3H-labeled C20:4 release, suggesting that utilization of this signaling pathway is specific for particular transformation stimuli.

Expression of the chicken vimentin gene in transgenic mice: efficient assembly of the avian protein into the cytoskeleton

To study expression and function of the vimentin gene, transgenic mice were generated by microinjecting the entire chicken gene plus 2.4 kilobases of 5' and 2.6 kilobases of 3' flanking sequences. All the transgenic mice obtained had incorporated multiple copies of the gene. RNA analyses demonstrated that the chicken vimentin gene was efficiently expressed in an appropriate tissue-specific pattern and that the transcripts were properly processed, as in chicken, giving rise to two RNAs. The vimentin transgene was predominantly expressed in lens at levels of up to 10-fold the endogenous level in every transgenic line studied. The chicken vimentin transcripts were efficiently translated into polypeptides that were modified posttranslationally and could assemble into the mouse cytoskeleton. Overexpression of the chicken vimentin gene did not obviously affect the expression of the endogenous gene at the RNA or the protein level. Immunofluorescence microscopy further demonstrated that the chicken protein was properly expressed spatially in lens. However, the levels were much higher in the transgenic animals.

Down-regulation of vimentin gene expression during myogenesis is controlled by a 5'-flanking sequence

During myogenesis, the intermediate filament proteins vimentin and desmin are differentially expressed. While desmin levels increase dramatically, vimentin mRNA levels decrease substantially. Here, we show that transfected whole- and mini-vimentin-coding genes (Vim) are expressed in fibroblasts (mouse L cells) and down-regulated during muscle cell differentiation in culture. Functional assays with 5'-end Vim::cat constructs demonstrate that this repression is controlled by a 5'-element (nt -321 to -160). This region is distinct from Vim promoter elements (nt -160 to +71) which do not contribute to vimentin's down-regulation during myogenesis.

78-kilodalton glucose-regulated protein is induced in Rous sarcoma virus-transformed cells independently of glucose deprivation

To identify mRNAs with altered expression in Rous sarcoma virus (RSV)-transformed cells, we screened a chicken embryo fibroblast (CEF) cDNA library by differential hybridization. One clone, designated R1H, showed markedly elevated mRNA expression in RSV-transformed cells. Nucleotide sequence analysis indicated that R1H mRNA encodes 78-kilodalton glucose-regulated protein (GRP78). Chicken GRP78 was found to be very highly conserved in comparison with rat GRP78 (96% identity between chicken and rat amino acid sequences). In contrast to previous observations, we found that GRP78 was induced in RSV-transformed cells in the absence of glucose deprivation. When cells were grown in glucose-supplemented medium, the level of GRP78 mRNA was approximately fivefold higher in RSV-transformed CEF than in transformation-defective virus-infected or uninfected CEF. Similar changes in GRP78 protein content were also found. Using a temperature-sensitive mutant of RSV and supplemental glucose, we found a gradual increase in the level of GRP78 mRNA beginning at 4 h after shiftdown to permissive temperature. Uridine supplementation did not block the induction seen in CEF infected with a temperature-sensitive mutant. These results indicate that GRP78 is induced by p60v-src in the absence of glucose deprivation.

Multiple elements are required for expression of an intermediate filament gene

The expression of vimentin is unique within the intermediate filament multigene family. It is the only member which deviates from its usual tissue-specific expression pattern and whose 5'-flanking region contains multiple GC boxes, the binding site for Sp1. The activity of vimentin 5'-end:CAT fusions has been compared in cells where vimentin is highly expressed (mouse L cells) or not expressed at all (MH1C1). In addition, CAT activity has been examined by microinjection into Xenopus oocytes. Both in vivo expression and in vitro binding studies implicate Sp1 as a general regulatory factor in vimentin gene expression. Increased expression of 5'-end:CAT fusions in mouse L cells suggests that a fibroblast-specific enhancer element resides in the region -321 to -160. Low transcriptional activity in MH1C1 cells may be due to either the lack of this positive transcription factor(s) or the presence of a repressor element. Here, we demonstrate that the unique and complex pattern of vimentin gene expression is controlled by multiple cis-acting elements.

78-kilodalton glucose-regulated protein is induced in Rous sarcoma virus-transformed cells independently of glucose deprivation

To identify mRNAs with altered expression in Rous sarcoma virus (RSV)-transformed cells, we screened a chicken embryo fibroblast (CEF) cDNA library by differential hybridization. One clone, designated R1H, showed markedly elevated mRNA expression in RSV-transformed cells. Nucleotide sequence analysis indicated that R1H mRNA encodes 78-kilodalton glucose-regulated protein (GRP78). Chicken GRP78 was found to be very highly conserved in comparison with rat GRP78 (96% identity between chicken and rat amino acid sequences). In contrast to previous observations, we found that GRP78 was induced in RSV-transformed cells in the absence of glucose deprivation. When cells were grown in glucose-supplemented medium, the level of GRP78 mRNA was approximately fivefold higher in RSV-transformed CEF than in transformation-defective virus-infected or uninfected CEF. Similar changes in GRP78 protein content were also found. Using a temperature-sensitive mutant of RSV and supplemental glucose, we found a gradual increase in the level of GRP78 mRNA beginning at 4 h after shiftdown to permissive temperature. Uridine supplementation did not block the induction seen in CEF infected with a temperature-sensitive mutant. These results indicate that GRP78 is induced by p60v-src in the absence of glucose deprivation.

Expression of transfected vimentin genes in differentiating murine erythroleukemia cells reveals divergent cis-acting regulation of avian and mammalian vimentin sequences

We studied the expression of transfected chicken and hamster vimentin genes in murine erythroleukemia (MEL) cells. MEL cells normally repress the levels of endogenous mouse vimentin mRNA during inducermediated differentiation, resulting in a subsequent loss of vimentin filaments. Expression of vimentin in differentiating MEL cells reflects the disappearance of vimentin filaments during mammalian erythropoiesis in vivo. In contrast, chicken erythroid cells express high levels of vimentin mRNA and vimentin filaments during terminal differentiation. We demonstrate here that chicken vimentin mRNA levels increase significantly in differentiating transfected MEL cells, whereas similarly transfected hamster vimentin genes are negatively regulated. In conjunction with in vitro nuclear run-on transcription experiments, these results suggest that the difference in vimentin expression in avian and mammalian erythropoiesis is due to a divergence of cis-linked vimentin sequences that are responsible for transcriptional and posttranscriptional regulation of vimentin gene expression. Transfected chicken vimentin genes produce functional vimentin protein and stable vimentin filaments during MEL cell differentiation, further demonstrating that the accumulation of vimentin filaments is determined by the abundance of newly synthesized vimentin.

Nucleotide sequence of the human vimentin gene and regulation of its transcription in tissues and cultured cells

We have previously reported the identification and isolation by mRNA selection/translation of a recombinant clone containing 80% of the human vimentin gene sequence [Lilienbaum et al., EMBO J. 5 (1986) 2809-2814]. We present here the nucleotide sequence of this genomic clone including the 3' untranslated region. To complete the coding sequence, we have isolated cDNA recombinant clones (1.1 kb) of vimentin from human libraries constructed in lambda gt11. Comparison of the coding sequence between human and hamster shows 90% homology at the nucleotide level and four differences out of 353 amino acid residues, as deduced from the nucleotide sequences. In addition to the extensive homology previously reported between the coding sequences of hamster and human vimentin genes [Ferrari et al., Mol. Cell. Biol. 6 (1986) 3614-3620], we observed that the positions of the noncoding regions are also conserved and that the 3' nontranslated region includes two canonic poly(A) signals. Hybridization of the clones to mRNA from different mammalian sources revealed a single species of 2 kb and confirmed that the length of the untranslated and coding sequences are conserved. Quantitative estimations of the mRNA levels in mammalian cells and tissues of various origins are consistent with transcriptional regulation.

Expression of transfected vimentin genes in differentiating murine erythroleukemia cells reveals divergent cis-acting regulation of avian and mammalian vimentin sequences

We studied the expression of transfected chicken and hamster vimentin genes in murine erythroleukemia (MEL) cells. MEL cells normally repress the levels of endogenous mouse vimentin mRNA during inducermediated differentiation, resulting in a subsequent loss of vimentin filaments. Expression of vimentin in differentiating MEL cells reflects the disappearance of vimentin filaments during mammalian erythropoiesis in vivo. In contrast, chicken erythroid cells express high levels of vimentin mRNA and vimentin filaments during terminal differentiation. We demonstrate here that chicken vimentin mRNA levels increase significantly in differentiating transfected MEL cells, whereas similarly transfected hamster vimentin genes are negatively regulated. In conjunction with in vitro nuclear run-on transcription experiments, these results suggest that the difference in vimentin expression in avian and mammalian erythropoiesis is due to a divergence of cis-linked vimentin sequences that are responsible for transcriptional and posttranscriptional regulation of vimentin gene expression. Transfected chicken vimentin genes produce functional vimentin protein and stable vimentin filaments during MEL cell differentiation, further demonstrating that the accumulation of vimentin filaments is determined by the abundance of newly synthesized vimentin.

Two ornithine decarboxylase mRNA species in mouse kidney arise from size heterogeneity at their 3' termini

Ornithine decarboxylase (OrnDCase; L-ornithine carboxy-lyase, EC 4.1.1.17) mRNA present in mouse kidney comprises two species with molecular sizes of approximately 2.2 and approximately 2.7 kilobases (kb). cDNA clones prepared from murine kidney OrnDCase mRNA were used to determine the reason for the size heterogeneity of these mRNAs. Two of the cDNA clones (pODC16 and pODC74) that differed at the 3' termini were isolated and sequenced. DNA sequencing indicated that each cDNA had a poly(A) tail; however, pODC74 was 429 nucleotides longer than pODC16 at the 3' end and contained two AATAAA signals for poly(A) addition. That the longer cDNA corresponded to the larger mRNA was confirmed by hybridization of a unique Pst I/Pst I fragment from the 3' terminus of pODC74 only to the 2.7-kb OrnDCase mRNA. The two cDNAs did not represent full-length copies of OrnDCase mRNAs and were 1199 (pODC16) and 1204 base pairs (bp) (pODC74) long. There were five mismatches in their 759-bp-long overlapping nucleotide sequence, suggesting that the 2.2- and 2.7-kb OrnDCase mRNAs may be products of two separate, yet very similar, OrnDCase genes. Androgen regulation of the accumulation of these two OrnDCase mRNAs appeared to occur coordinately, as testosterone administration brought about comparable increases in their concentrations in mouse kidney.

The chicken vimentin gene: aspects of organization and transcription during myogenesis

The intermediate filament gene vimentin exists in a single copy in the chick haploid genome. However, it exhibits the curious property of producing at least three functional vimentin mRNA transcripts in vivo through the differential utilization of multiple polyadenylylation sites. According to one group in erythroid cells there may be a tissue-specific utilization of one of these poly A addition sites. The chicken and hamster vimentin genes exhibit remarkable nucleotide sequence homology both within coding and 3'-noncoding regions (82%). This nucleotide homology extends both to the size and juxtaposition of exons. With the noted exception of valine, even the frequency of codons utilized is strongly conserved across the widely different species. Of course, this strong homology at the DNA level extends to an amino acid homology of 92% between vimentins and 65% between related proteins in the same species.

Translational activity of mRNA coding for cytoskeletal brain proteins in newborn and adult mice: a comparative study

Translational activity of mRNA coding for cytoskeletal brain proteins was used to determine the relative abundance of the mRNA in the brains of newborn and adult mice. mRNA was translated in a cell-free system containing rabbit reticulocyte factors. The products of translation were analyzed by two-dimensional gel electrophoresis and characterized by peptide map analysis. Comparison of the products of translation from newborn and from adult brain mRNA shows a 50% decrease in actin and tubulin from newborn to the adult stage. In contrast, the 70 kd neurofilament protein and glial fibrillary acidic protein show a twofold increase in the adult stage. The heat-shock protein HSP70 increases slightly (30%) whereas the brain isozyme of creatine kinase and the heat-shock protein HSP90 are three times as high in adult subjects as in newborns.

Multiple polyadenylation sites in a Drosophila tropomyosin gene are used to generate functional mRNAs

The gene encoding muscle tropomyosin I in Drosophila is alternatively spliced in embryonic and thoracic muscle to generate two sizes classes of RNAs. By Northern blot analysis, the embryonic RNA class shows a broad RNA band of hybridization of 1.3 kb and a more sharply defined, less abundant RNA band at 1.6 kb. The thoracic class of RNAs, on the other hand, consists of a broad hybridization band at 1.7 kb and a more sharply defined band at 1.9 kb. Each size class of RNA encodes a different tropomyosin isoform. The two classes of alternatively spliced RNAs utilize the same 3' terminal exon of the gene. The DNA sequence of this exon reveals a cluster of several polyadenylation signals (AAUAAA) or polyadenylation-like signals. We show here by S1 nuclease protection analysis that at least five and possibly seven of these polyadenylation or polyadenylation-like sequences are associated with in vivo embryonic and thoracic mRNA cleavage processing sites. Six of these S1 sites are clustered within 119 bp and a seventh is located 255 bp downstream. At least one of the polyadenylation-like signal sequences appears to be an unusual AACAAA sequence. In addition we also show that these mRNAs function in vitro to synthesize muscle tropomyosins.