Transient induction of poly(A)-short myosin heavy chain messenger RNA during terminal differentiation of L6E9 myoblasts

Space shuttle flight (STS-45) of L8 myoblast cells results in the isolation of a nonfusing cell line variant

Myoblast cell cultures have been widely employed in conventional (1g) studies of biological processes because characteristics of intact muscle can be readily observed in these cultured cells. We decided to investigate the effects of spaceflight on muscle by utilizing a well characterized myoblast cell line (L8 rat myoblasts) as cultured in the recently designed Space Tissue Loss Flight Module "A" (STL-A). The STL-A is a "state of the art," compact, fully contained, automated cell culture apparatus which replaces a single mid-deck locker on the Space Shuttle. The L8 cells were successfully flown in the STL-A on the Space Shuttle STS-45 mission. Upon return to earth, reculturing of these spaceflown L8 cells (L8SF) resulted in their unexpected failure to fuse and differentiate into myotubes. This inability of the L8SF cells to fuse was found to be a permanent phenotypic alteration. Scanning electron microscopic examination of L8SF cells growing at 1g on fibronectin-coated polypropylene fibers exhibited a strikingly different morphology as compared to control cells. In addition to their failure to fuse into myotubes, L8SF cells also piled up on top of each other. When assayed in fusion-promoting soft agar, L8SF cells gave rise to substantially more and larger colonies than did either preflight (L8AT) or ground control (L8GC) cells. All data to this point indicate that flying L8 rat myoblasts on the Space Shuttle for a duration of 7-10 d at subconfluent densities results in several permanent phenotypic alterations in these cells.

Lateral motion of fluorescent molecules embedded into cell membranes of clonal myogenic cells, L6, changes upon cell maturation

The lateral motion of fluorescent molecules embedded into cell membranes of myogenic cell line, L6, was measured. The motion of S-F-ConA became faster at cell fusion stage, and became slower after fusion. On the other hand, the motion of lipid analog, F18, was not changed at cell fusion stage. However, after fusion when myotubes were formed, the motion of F18 became slower. At cell fusion stage, there was a large variation in the motion of S-F-ConA. This means that at this stage the properties of myoblasts change drastically and rapidly.

Multiple positive and negative 5' regulatory elements control the cell-type-specific expression of the embryonic skeletal myosin heavy-chain gene

To identify the DNA sequences that regulate the expression of the sarcomeric myosin heavy-chain (MHC) genes in muscle cells, a series of deletion constructs of the rat embryonic MHC gene was assayed for transient expression after introduction into myogenic and nonmyogenic cells. The sequences in 1.4 kilobases of 5'-flanking DNA were found to be sufficient to direct expression of the MHC gene constructs in a tissue-specific manner (i.e., in differentiated muscle cells but not in undifferentiated muscle and nonmuscle cells). Three main distinct regulatory domains have been identified: (i) the upstream sequences from positions -1413 to -174, which determine the level of expression of the MHC gene and are constituted of three positive regulatory elements and two negative ones; (ii) a muscle-specific regulatory element from positions -173 to -142, which restricts the expression of the MHC gene to muscle cells; and (iii) the promoter region, downstream from position -102, which directs transcription initiation. Introduction of the simian virus 40 enhancer into constructs where subportions of or all of the upstream sequences are deleted (up to position -173) strongly increases the level of expression of such truncated constructs but without changing their muscle specificity. These upstream sequences, which can be substituted for by the simian virus 40 enhancer, function in an orientation-, position-, and promoter-dependent fashion. The muscle-specific element is also promoter specific but does not support efficient expression of the MHC gene. The MHC promoter in itself is not muscle specific. These results underline the importance of the concerted action of multiple regulatory elements that are likely to represent targets for DNA-binding-regulatory proteins.

Multiple positive and negative 5' regulatory elements control the cell-type-specific expression of the embryonic skeletal myosin heavy-chain gene

To identify the DNA sequences that regulate the expression of the sarcomeric myosin heavy-chain (MHC) genes in muscle cells, a series of deletion constructs of the rat embryonic MHC gene was assayed for transient expression after introduction into myogenic and nonmyogenic cells. The sequences in 1.4 kilobases of 5'-flanking DNA were found to be sufficient to direct expression of the MHC gene constructs in a tissue-specific manner (i.e., in differentiated muscle cells but not in undifferentiated muscle and nonmuscle cells). Three main distinct regulatory domains have been identified: (i) the upstream sequences from positions -1413 to -174, which determine the level of expression of the MHC gene and are constituted of three positive regulatory elements and two negative ones; (ii) a muscle-specific regulatory element from positions -173 to -142, which restricts the expression of the MHC gene to muscle cells; and (iii) the promoter region, downstream from position -102, which directs transcription initiation. Introduction of the simian virus 40 enhancer into constructs where subportions of or all of the upstream sequences are deleted (up to position -173) strongly increases the level of expression of such truncated constructs but without changing their muscle specificity. These upstream sequences, which can be substituted for by the simian virus 40 enhancer, function in an orientation-, position-, and promoter-dependent fashion. The muscle-specific element is also promoter specific but does not support efficient expression of the MHC gene. The MHC promoter in itself is not muscle specific. These results underline the importance of the concerted action of multiple regulatory elements that are likely to represent targets for DNA-binding-regulatory proteins.

Transcriptional and posttranscriptional control of c-myc during myogenesis: its mRNA remains inducible in differentiated cells and does not suppress the differentiated phenotype

It is widely accepted that the cellular oncogene c-myc plays an important role in the control of cell proliferation and that its expression diminishes in differentiated cells. We examined whether there is a correlation between c-myc expression and cell proliferation or differentiation by using a subclone of a rat skeletal muscle cell line L6E9. Myoblasts irreversibly withdraw from the cell cycle, fuse to form multinucleated myotubes, and express muscle-specific genes (terminal differentiation). Muscle-specific genes can also be expressed in the absence of fusion (biochemical differentiation). Such mononucleated but biochemically differentiated cells can be stimulated to reenter the cell cycle. c-myc was induced by insulin, insulin-like growth factor, or serum factors in G0-arrested cells, whereas induction by protein synthesis inhibitors or superinduction by protein synthesis inhibitors in combination with serum factors occurred in all physiological states tested. We found that c-myc expression was reduced in biochemically and terminally differentiated cells as well as in quiescent undifferentiated cells but that it remained inducible by growth factors in all three physiological states. Results of nuclear runoff transcription assays suggested that the induction of c-myc mRNA by growth factors and its deinduction in these physiological states were regulated mainly at the transcriptional level. In contrast, induction and superinduction of c-myc mRNA by protein synthesis inhibitors alone and in combination with growth factors, respectively, were regulated posttranscriptionally mainly by stabilization of c-myc mRNA. Moreover, c-myc and muscle-specific genes could be simultaneously transcribed in both biochemically and terminally differentiated cells. These results indicate that irreversible repression of c-myc is not required for terminal myogenic differentiation and that its expression is insufficient by itself to suppress the differentiated phenotype.

Type beta transforming growth factor is an inhibitor of myogenic differentiation

We have investigated the effect of type beta transforming growth factor (TGF-beta) on the differentiation of skeletal muscle myoblasts. TGF-beta potently (ID50 approximately 10 pM) prevents established cell lines and primary cultures of rat and chicken embryo myoblasts from fusing into multinucleated myotubes. Inhibition of morphological differentiation by TGF-beta correlates with inhibition of the expression of muscle-specific mRNAs and proteins, strong induction of extracellular matrix type I collagen and fibronectin, and a marked tendency of the treated myoblasts to aggregate into densely multilayered arrays or clusters. Myogenic differentiation can resume after removal of TGF-beta from the medium. Examination of the time of action of TGF-beta shows that myoblasts stochastically reach a point beyond which they become insensitive to the inhibitory action of TGF-beta. This resistance of committed myoblasts to the inhibitory action of TGF-beta is not associated with any measurable change in the number or affinity of TGF-beta receptors in those cells. The results indicate that TGF-beta is a potent inhibitor of myogenesis and may regulate muscle development in vivo.

Complete nucleotide and encoded amino acid sequence of a mammalian myosin heavy chain gene. Evidence against intron-dependent evolution of the rod

The complete nucleotide sequence and exon/intron structure of the rat embryonic skeletal muscle myosin heavy chain (MHC) gene has been determined. This gene comprises 24 X 10(3) bases of DNA and is split into 41 exons. The exons encode a 6035 nucleotide (nt) long mRNA consisting of 90 nt of 5' untranslated, 5820 nt of protein coding and 125 nt of 3' untranslated sequence. The rat embryonic MHC polypeptide is encoded by exons 3 to 41 and contains 1939 amino acid residues with a calculated Mr of 223,900. Its amino acid sequence displays the structural features typical for all sarcomeric MHCs, i.e. an amino-terminal "globular" head region and a carboxy-terminal alpha-helical rod portion that shows the characteristics of a coiled coil with a superimposed 28-residue repeat pattern interrupted at only four positions by "skip" residues. The complex structure of the rat embryonic MHC gene and the conservation of intron locations in this and other MHC genes are indicative of a highly split ancestral sarcomeric MHC gene. Introns in the rat embryonic gene interrupt the coding sequence at the boundaries separating the proteolytic subfragments of the head, but not at the head/rod junction or between the 28-residue repeats present within the rod. Therefore, there is little evidence for exon shuffling and intron-dependent evolution by gene duplication as a mechanism for the generation of the ancestral MHC gene. Rather, intron insertion into a previously non-split ancestral MHC rod gene consisting of multiple tandemly arranged 28-residue-encoding repeats, or convergent evolution of an originally non-repetitive ancestral MHC rod gene must account for the observed structure of the rod-encoding portion of present-day MHC genes.

Transcriptional and posttranscriptional control of c-myc during myogenesis: its mRNA remains inducible in differentiated cells and does not suppress the differentiated phenotype

It is widely accepted that the cellular oncogene c-myc plays an important role in the control of cell proliferation and that its expression diminishes in differentiated cells. We examined whether there is a correlation between c-myc expression and cell proliferation or differentiation by using a subclone of a rat skeletal muscle cell line L6E9. Myoblasts irreversibly withdraw from the cell cycle, fuse to form multinucleated myotubes, and express muscle-specific genes (terminal differentiation). Muscle-specific genes can also be expressed in the absence of fusion (biochemical differentiation). Such mononucleated but biochemically differentiated cells can be stimulated to reenter the cell cycle. c-myc was induced by insulin, insulin-like growth factor, or serum factors in G0-arrested cells, whereas induction by protein synthesis inhibitors or superinduction by protein synthesis inhibitors in combination with serum factors occurred in all physiological states tested. We found that c-myc expression was reduced in biochemically and terminally differentiated cells as well as in quiescent undifferentiated cells but that it remained inducible by growth factors in all three physiological states. Results of nuclear runoff transcription assays suggested that the induction of c-myc mRNA by growth factors and its deinduction in these physiological states were regulated mainly at the transcriptional level. In contrast, induction and superinduction of c-myc mRNA by protein synthesis inhibitors alone and in combination with growth factors, respectively, were regulated posttranscriptionally mainly by stabilization of c-myc mRNA. Moreover, c-myc and muscle-specific genes could be simultaneously transcribed in both biochemically and terminally differentiated cells. These results indicate that irreversible repression of c-myc is not required for terminal myogenic differentiation and that its expression is insufficient by itself to suppress the differentiated phenotype.

Regulation of creatine kinase induction in differentiating mouse myoblasts

The regulation of creatine kinase (CK) induction during muscle differentiation was analyzed with MM14 mouse myoblasts. These cells withdraw from the cell cycle and commit to terminal differentiation when fed with mitogen-depleted medium. Myoblasts contained trace amounts of an isozyme of brain CK (designated BB-CK), but differentiation was accompanied by the induction of two other isozymes of muscle and brain CKs (designated MM-CK and MB-CK). Increased CK activity was detectable within 6 h of mitogen removal, 3 h after the first cells committed to differentiation and 6 h before fusion began. By 48 h, MM-CK activity increased more than 400-fold, MB-CK activity increased more than 150-fold, and BB-CK activity increased more than 10-fold. Antibodies prepared against purified mouse MM-CK cross-reacted with muscle and brain CKs (designated M-CK and B-CK, respectively) from a variety of species and were used to demonstrate that the increase in enzymatic activity was paralleled by an increase in the protein itself. CK antibodies were also used to aid in identifying cDNA clones to M-CK. cDNA sequences which corresponded to protein-coding regions cross-hybridized with B-CK mRNA; however, a subclone containing the 3'-nontranslated region was unique and was used to quantitate M-CK mRNA levels during myoblast differentiation. M-CK mRNA was not detectable in myoblasts, but within 5 to 6 h of mitogen withdrawal (6 to 7 h before fusion begins) it accumulated to about 30 molecules per cell. By 24 h, myotubes contained approximately 1,100 molecules per nucleus of M-CK mRNA.

Regulation of creatine kinase induction in differentiating mouse myoblasts

The regulation of creatine kinase (CK) induction during muscle differentiation was analyzed with MM14 mouse myoblasts. These cells withdraw from the cell cycle and commit to terminal differentiation when fed with mitogen-depleted medium. Myoblasts contained trace amounts of an isozyme of brain CK (designated BB-CK), but differentiation was accompanied by the induction of two other isozymes of muscle and brain CKs (designated MM-CK and MB-CK). Increased CK activity was detectable within 6 h of mitogen removal, 3 h after the first cells committed to differentiation and 6 h before fusion began. By 48 h, MM-CK activity increased more than 400-fold, MB-CK activity increased more than 150-fold, and BB-CK activity increased more than 10-fold. Antibodies prepared against purified mouse MM-CK cross-reacted with muscle and brain CKs (designated M-CK and B-CK, respectively) from a variety of species and were used to demonstrate that the increase in enzymatic activity was paralleled by an increase in the protein itself. CK antibodies were also used to aid in identifying cDNA clones to M-CK. cDNA sequences which corresponded to protein-coding regions cross-hybridized with B-CK mRNA; however, a subclone containing the 3'-nontranslated region was unique and was used to quantitate M-CK mRNA levels during myoblast differentiation. M-CK mRNA was not detectable in myoblasts, but within 5 to 6 h of mitogen withdrawal (6 to 7 h before fusion begins) it accumulated to about 30 molecules per cell. By 24 h, myotubes contained approximately 1,100 molecules per nucleus of M-CK mRNA.

Variation of the poly(A) size classes in the rat mammary gland during the lactation cycle

The sizes of the poly(A) tracts associated with rat mammary RNA were determined at several time points in the lactation cycle. The poly(A) tracts in the lactating gland displayed two predominant size class peaks at 80-85 and 45-47 residues. The 9S whey protein mRNA and the 15S casein mRNA purified from the 12 day lactating mammary gland both contained poly(A) tracts displaying a similar size distribution. The 45 residue tracts were a characteristic of lactation; they were not found at 8 days of pregnancy and only small amounts of these shorter poly(A) tracts were found in the 16 day pregnant gland. The poly(A) tracts of the involuted gland displayed the same size characteristics as those of late pregnancy. At all the developmental stages that were examined, the fraction of 45 residue poly(A) tracts was always proportional to the total poly(A) content of the mammary cells.

Developmental changes in chromatin of skeletal muscle of rats: acetylation of chromosomal proteins and transcription

In vivo acetylation of chromosomal proteins and RNA synthesis were studied in the skeletal muscle of 3-30 day old developing rats. The levels of acetylated histones and nonhistone chromosomal (NHC) proteins are high at day 3 and decrease as development progresses. Spermine has no significant effect on acetylation. Incorporation of 3H-Uridine into RNA of both nuclear and cytoplasmic fractions is also maximum at day 3 and plateaus by day 14. Nuclear RNA synthesis following acetylation of chromosomal proteins is greatly stimulated in all the ages studied, whereas that in cytoplasmic RNA occurs only in 3 day old rats. Such modifications during early development may bring about conformational and functional changes in the chromatin and contribute significantly to the process of terminal differentiation of skeletal muscle cells.

A transient species of poly(A)+ RNA detected by a myosin heavy chain cDNA probe in muscle cell culture during terminal differentiation

Primary cell cultures were prepared from breast muscles of 11 day 4 hour-embryonic chicks. Cytoplasmic RNAs were isolated from the cultured cells at various time intervals from day 3 to day 8. A [P32] DNA probe complementary to messenger RNA of myosin heavy chain was used to hybridize with the RNAs after gel electrophoresis. A transient species of polyadenylated RNA with a decreased mobility in electrophoresis was detected during a period of time when contractions of syncytial fibers were first observed.

Alpha actin gene exist in an active structural configuration in the proliferating myoblasts as well as in differentiated myotubes of the L6 line

Single-stranded DNA (ssDNA) mainly consisting of transcription sites was probed with cDNA actin clones for studying the expression of actin specific genes during myogenesis in the L6 line of rat myogenic cells. As compared with total nuclear DNA, ssDNA from myoblasts and myotubes was found greatly enriched in sequences complementary to both muscular and non muscular actin sequences. In contrast, ssDNA from spleen, hepatocytes or hepatoma cells was found enriched only in sequences complementary to non muscle actin cDNA. Actin specific sequences accumulated in the ssDNA fraction are almost entirely derived from the coding DNA strand. The DNAase I sensitivity of the actin genes sequences in myogenic and non myogenic cells correlated the data obtained with the ssDNA fraction. It is concluded that the muscle specific actin genes is either transcriptionally active or at least exist in an active configuration in the proliferating myoblasts as well as in the terminally differentiated myotubes.

Inhibition of muscle differentiation by trypanosoma cruzi

L6E9 rat myoblasts were infected in tissue culture with the myotropic Brazil strain of Trypanosoma cruzi. The effect of parasite infection on the ability of myoblasts to differentiate into myotubes was studied. Both morphological and biochemical differentiation were found to be profoundly affected by parasitic infection in a dose-related fashion. Evidence is presented to suggest that infected myoblasts can no longer differentiate. Differentiation, once underway, seemed unaffected by the parasitic infection; biochemical markers of differentiation remained intact.

Reversibility of muscle differentiation in the absence of commitment: analysis of a myogenic cell line temperature-sensitive for commitment

The interrelationship between commitment (irreversible withdrawal from the cell cycle) and muscle-specific gene expression was analyzed with the myogenic cell line ts 3b-2, which is temperature sensitive for commitment and cell fusion. The rates of synthesis and levels of accumulation of muscle-specific mRNAs and proteins in the ts 3b-2 cells at permissive and nonpermissive temperatures are comparable, indicating that neither commitment nor cell fusion is required for induction of muscle-specific gene expression. In the absence of commitment, the cells are reversibly withdrawn from the cell cycle during gene induction, and expression of the muscle-specific genes is deinduced upon the switch to growth-stimulating conditions. The deinduction reflects coordinate and preferential cessation of muscle-specific mRNA synthesis, coupled with destabilization of the muscle-specific mRNAs in the cytoplasm, without effect on constitutively expressed housekeeping protein genes. The phenotype of the ts 3b-2 cells demonstrates that commitment and muscle-specific gene expression are both required, but alone are insufficient, to produce the terminally differentiated muscle phenotype.

Effect of sodium butyrate on messenger RNA populations in myogenic cells in culture

Sodium butyrate, when added in millimolar concentrations to a culture of myoblasts of the L6 cell line, inhibits reversibly cell proliferation and differentiation. The aim of this work was to study the effect of sodium butyrate on the nuclear and cytoplasmic RNAs in these cells. We have prepared (3H) DNAs complementary to cytoplasmic polyadenylated RNAs from treated and untreated cells and performed homologous and heterologous hybridizations with cytoplasmic polyadenylated RNAs and with total nuclear RNAs. The hybridization kinetics led to the following conclusions: a) Hybridization with nuclear RNAs shows that butyrate allows the transcription of most of the RNA sequences synthesized in proliferating myoblasts, including the sequences that are no longer synthesized in untreated myotubes. However some differences in the hybridization saturation levels indicate that sodium butyrate might modify the expression of a limited number of genes involved in cell proliferation, muscular differentiation, or both. b) Hybridization with cytoplasmic polyadenylated RNAs shows that sodium butyrate acts also at a post-transcriptional level, it produces a large decrease in the frequency of the abundant sequences present in untreated cells, but has little effect on the total number of different RNA species.

Sarcomeric myosin heavy chain is coded by a highly conserved multigene family

pMHC25, a recombinant plasmid containing myosin heavy chain (MHC) cDNA sequences from differentiated myotubes of the L6E9 rat cell line, has been shown to hybridize to all sarcomeric MHC mRNAs so far tested but not to nonsarcomeric MHC mRNAs. In addition, pMHC25 hybridizes to multiple restriction endonuclease-digested fragments of rat genomic DNA corresponding to different MHC genomic sequences. Thus, the MHC gene represented by pMHC25 is a member of a sarcomeric MHC multigene family that has regions of sequence homology shared among its members. This sarcomeric MHC multigene family has been estimated to be composed of a minimum of seven genes, some of which are polymorphic in the rat. We have also determined that pMHC25 hybridizes to MHC gene sequences in genomic DNA of all species that have striated muscle, ranging from nematodes to man. Sarcomeric MHC genes, therefore, have been horizontally and vertically conserved in evolution. Additionally, we have used the pMHC25 plasmid to demonstrate that MHC genes do not undergo rearrangement or amplification during muscle cell differentiation.

Nonadenylylated mRNA is present as polyadenylylated RNA in nuclei of Drosophila

The sequence complexity of nuclear total RNA and nuclear poly(A)+RNA from Drosophila third-instar larvae was determined by hybridization of these RNAs to labeled single-copy DNA. At saturation, the nuclear poly(A)+ - and total RNA hybridized to 11% and 22.5% of the single-copy DNA, respectively. The increase in complexity of nuclear total RNA over that observed for nuclear poly(A)+RNA indicates the presence of a discrete class of nonoadenylylated nuclear RNA molecules. The relationship between DNA sequences coding for nuclear RNA and mRNA was then determined by hybridization of nuclear total and poly(A)+RNA to DNA enriched for mRNA coding sequences. The results of these studies show that those single-copy DNA sequences that are represented in either the poly(A)+ - or poly(A)- mRNA population are transcribed into RNA molecules that appear in the nuclear poly(A)+RNA population.

Gene switching in myogenesis: differential expression of the chicken actin multigene family

We described the construction of an alpha-actin complementary deoxyribonucleic acid (cDNA) clone, pAC269 [Schwartz, R. J., Haron, J. A., Rothblum, K. N., & Dugaiczyk, A. (1980) Biochemistry 19, 5883], that was used as a hybridization probe in the current investigation to examine the induction of actin messenger ribonucleic acid (mRNA) during myogenesis. A Tm difference of 10-13 degrees C between skeletal muscle alpha-actin and nonmuscle beta- and gamma-actin mRNAs and pAC269 allowed us to establish the highly stringent hybridization conditions necessary to measure separately the content of alpha-actin mRNA and beta- and gamma-actin mRNA during muscle development in culture. We observed low levels of alpha-actin mRNA (approximately 130 molecules/cell) in replicating prefusion myoblasts. The vast majority of actin mRNA (2000 molecules/cell) present at this stage was accounted for by beta- and gamma-actin mRNA. Beginning at myoblast fusion, alpha-actin mRNA accumulated and within 30 h reached a level 270-fold greater than that observed in the undifferentiated state. At 95 h in culture when myotube formation was completed, alpha-actin content was at its peak (36 000 molecules/nucleus). Conversely, beta- and gamma-actin mRNA content began to decline at the beginning of fusion, and by the end of myotube formation beta- and gamma-actin mRNAs were undetectable by our techniques. A rapid depression of alpha-actin mRNA levels was observed after 95 h in the absence of cell death. At 6 days after the initiation of myotube formation, the content of alpha-actin mRNA was reduced by 80% in comparison of peak values and remained at that level. The switching of actin mRNA species was inhibited in myoblasts treated with bdU. The accumulation of alpha-actin mRNA and the disappearance of beta- and gamma-actin mRNA were observed following the reversal of the bdU block and coincident with the onset of myoblast fusion. We found that the expression of actin genes within the actin multigene family is switched in myogenesis through a strict developmental pattern.

Cytoplasmic processing of myosin heavy chain messenger RNA: evidence provided by using a recombinant DNA plasmid

A recombinant DNA plasmid, designated pMHC25, has been constructed that contains structural gene sequences for rat skeletal muscle myosin heavy chain (MHC). The identity of the MHC sequence insert in pMHC25 was determined by muscle-tissue specificity, inhibition of MHC protein synthesis in vitro by hybrid-arrested translation, purification of mRNA that directs the synthesis of MHC protein in vitro, and hybridization to a 33S cytoplasmic mRNA found only in differentiated muscle cells. pMHC25-DNA-excess filter hybridizations were used to show that more than 90% of the newly synthesized MHC mRNA that appears in the cytoplasm of differentiated L6E9 myotubes contains a long 3' poly(A) tail. In contrast, 90% of the MHC mRNA that accumulates in the cytoplasm of these same cells during myogenic differentiation lacks this long 3' poly(A) tail. These results suggest the occurrence of a posttranscriptional event in differentiated L6E9 myotubes that involves the cytoplasmic processing of poly(A)+ MHC mRNA to poly(A)- or poly(A)-short MHC mRNA.