Messenger RNA for myosin polypeptides: isolation from single myogenic cell cultures

Regulation of tropomyosin gene expression during myogenesis

In skeletal muscle, tropomyosin has a critical role in transduction of calcium-induced contraction. Presently, little is known about the regulation of tropomyosin gene expression during myogenesis. In the present study, qualitative and quantitative changes in the nucleic acid populations of differentiating chicken embryo muscle cells in culture have been examined. Total nucleic acid content per nucleus increased about fivefold in fully developed myotubes as compared to mononucleated myoblasts. The contribution of deoxyribonucleic acid to the total nucleic acid population decreased from 24% in myoblasts to 5% of total nucleic acid in myotubes. Concomitant with the decrement in deoxyribonucleic acid contribution to total nucleic acid was an increase in polyadenylated ribonucleic acid (RNA) content per cell which reached levels in myotubes that were 17-fold higher than those of myoblasts. Specific changes in the RNA population during myogenesis were further investigated by quantitation of the synthetic capacity (messenger RNA levels) per cell for alpha- and beta-tropomyosin. Cell-free translation and immunoprecipitation demonstrated an approximately 40-fold increase in messenger RNA levels per nucleus for alpha- and beta-tropomyosin after fusion in the terminally differentiated myotubes. Indirect immunofluorescence with affinity-purified tropomyosin antibodies demonstrated the presence of tropomyosin-containing filaments in cells throughout myogenesis. Thus, the tropomyosin genes are constitutively expressed during muscle differentiation through the production of tropomyosin messenger RNA and translation into tropomyosin protein.

Expression of J chain RNA in cell lines representing different stages of B lymphocyte differentiation

During B cell differentiation to pentamer IgM secretion, synthesis of the pentamer joining component, the J chain, is initiated. We investigated the mechanism for initiating J chain synthesis by analyzing murine cell lines representing different stages in B cell differentiation. The expression of functional J chain mRNA was evaluated by cell-free translation and specific immunoprecipitation of a J chain product. The expression of precursor mRNA was examined by hybridization with a J chain probe obtained by molecular cloning of cDNA. No J chain-specific RNA could be demonstrated in a lymphoma line representative of an undifferentiated B lymphocyte, but three species of J chain RNA were identified in hybrid cell lines representative of IgM-secreting plasma cells: a mature message of approximately 1.5 kb and two minor components of 2.5 and 0.92 kb. The encounter of a B cell with antigen or mitogen must therefore trigger events that effect either transcription of J chain sequences or their intranuclear stabilization.

Developmental regulation of elastin synthesis

The regulation of elastin production has been evaluated and compared in two systems in the developing sheep: nuchal ligament and lung. Absolute rates of production in explant culture were estimated by immunoprecipitation of newly synthesized elastin and determination of precursor pool specific activities. Lung elastin production increased about 2.5-fold while nuchal ligament production increased about 9-fold during the latter half of gestation. In comparison, the activities of elastin messenger RNA(mRNAE) of these tissues were determined in a cell-free system by immunoprecipitation. The increases in mRNAE activity largely paralleled the rises in elastin production, suggesting that elastin synthesis in these tissues is regulated, at least in part, by the availability of translationally active mRNAE.

The isolation of cloned cDNA sequences which are differentially expressed in human lymphocytes and fibroblasts

Poly(A)+ RNA populations derived from normal lymphocytes and fibroblasts have been compared by hybridising each RNA to cDNA derived from the other RNA population. This indicated that approximately 75% of the sequences were common to both, and that these were present at different concentrations in the two cell types. The two RNA populations were further compared by hybridising them to a cDNA recombinant library derived from lymphocyte poly(A)+ RNA. This allowed the identification of clones containing sequences which are abundant in lymphocyte poly(A)+ RNA but absent or rare in fibroblast poly(A)+ RNA. A direct estimation of the abundance of five of these sequences in lymphocyte cDNA demonstrated that clones can be detected by such a procedure if they represent 0.2% or greater of the original cDNA population.

Optimal conditions for cell-free synthesis of elastin

Optimal conditions for the translation of elastin mRNA1 in a mRNA-dependent rabbit reticulocyte lysate were determined. Using total RNA isolated from embryonic chick aortae as the source of exogenous RNA, the concentrations of various components present in the translation assay were varied and the effect on elastin synthesis quantitated by immunoprecipitation. Components examined included: magnesium acetate, potassium chloride, spermidine, creatine phosphate, ATP, and GTP. In addition, it was found that heating of the RNA prior to translation significantly enhanced total protein synthesis, elastin synthesis, and the synthesis of proteins possessing molecular weights of greater than 80,000.

The construction of a recombinant cDNA library representative of the poly(A)+ mRNA population from normal human lymphocytes

A recombinant library has been constructed using the plasmid pAT153 and double stranded cDNA prepared from normal human lymphocyte poly(A)+ RNA. Transformation conditions were optimized to yield approximately 200,000 recombinants per microgram of double stranded cDNA. Statistical analysis as well as sequence complexity analysis of the inserted sequences indicates that the cDNA library is representative of > 99% of the poly(A)+ RNA present in the normal human lymphocyte.

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.

The messenger RNA sequences in human fibroblast cells induced with poly rI.rC to produce interferon

Induction of human fibroblast cells with poly rI.rC induces interferon mRNA which can be translated into interferon precursor in wheat germ cell free system or in Xenopus oocytes into biologically active interferon. The extent of gene expression in the poly rI.rC induced cells was compared to that of the uninduced cells by hybridization of the mRNA to complementary DNA. Homologous template driven hybridization of cDNA revealed the presence of two clearly defined transitions in the total poly A RNA from the induced cells; abundant class and a scarce class comprising approximately 37,000 diverse species of RNA. Heterologus hybridization of the cDNA with total uninduced mRNA showed that the majority of the mRNA sequences are the same in both the induced and uninduced cells. The results of the hybridization using cDNA prepared to the fraction enriched for interferon mRNA, however, showed that about 4% of the sequences present in the interferon enriched fraction are not present in the uninduced cells. These differences may result from the poly rI.rC induced alterations in gene expression.

Activity of multiple light chain genes in murine myeloma cells producing a single, functional light chain

Two cloned lambda 1-producing myelomas (HOPC-1, MOPC-104E) contain rearranged kappa genes and levels of mature-sized kappa RNA comparable to those found in kappa-producing myeloma cells. Another lambda 1-producing myeloma tumor line (HOPC-2020) and a lambda 1-containing B cell leukemia line (BCL1) also contain significant levels of kappa RNA. One lambda 11-producing line (MOPC-315) contains no detectable kappa RNA, but it also has no kappa genes in the embryonic configuration. kappa-related proteins are not detectable in the lambda 1-producing lines by standard procedures, but by sensitive methods at least two lines contain kappa protein fragments. The MOPC-104E line produces both a 14.5K kappa fragment that is not readily detectable because of its low rate of synthesis and short half-life (T 1/2 less than 5 min), and a major 16.5K protein that lacks kappa cross reactivity but is demonstrable by translation of purified MOPC-104E kappa RNA. The HOPC-1 kappa RNA also encodes a short-lived 14K kappa fragment. The MPC-11 line, which produces a mature kappa RNA and protein as well as an 800 base kappa fragment RNA and kappa protein fragment, has both kappa alleles rearranged, one apparently aberrantly between J and C kappa. Two different kappa RNA species, one the same size as the MPC-11 kappa fragment RNA, frequently are present in kappa RNA-containing Abelson murine leukemia virus-transformed lymphoid cells as well as in 18 and 19 day murine fetal liver. For light chains, neither allelic nor isotype exclusion is generally evident in myeloma and lymphoma cells; rather both produce only a single functional light chain. Models of light chain activation must explain restriction by considering the functional properties of the light chain rather than light chain gene expression.

Molecular cloning of developmentally regulated, low-abundance mRNA sequences from embryonic muscle

To study the role of low-abundance, embryonic muscle-specific gene transcripts, we have developed a method to screen cDNA clones from embryonic muscle for such sequences. The protocol involves two stages: first, partial enrichment for cDNA clones carrying possible embryo-specific sequences by selecting clones of low-abundance sequences; and second, determination, by hybridization to RNA attached to diazobenzyloxymethyl-paper, which sequences from this category are regulated in an embryonic muscle-specific manner during development. At least three different clones were obtained which hybridized to sequences present in early muscle development but absent, or present at relatively low levels, at late embryonic and adult muscle stages. Two of these clones were not muscle-specific because they hybridized to poly(A)+RNA from liver or brain or both. The third clone, 106A4, did not detectably hybridize to total poly(A)+RNA at any stage of brain or liver development tested. This sequence also was not detectable in poly(A)+RNA from embryonic muscle progenitor cells. Thus, the 106A4 sequence is a likely candidate for an embryonic muscle-specific sequence. We have demonstrated that the 106A4 sequence is a mRNA, although the specific identity and function of the translated product is unknown. The method used to identify embryonic muscle-specific cDNA clones should be generally applicable for obtaining clones for low abundance transcripts regulated in a tissue-specific or developmental stage-specific manner.

Molecular mechanism for the capture and excision of the transforming gene of avian sarcoma virus as suggested by analysis of recombinant clones

Structural analysis of two cDNA clones, derived from reverse transcripts of avian sarcoma virus 21S mRNA's, reveals unusual features in the organization and expression of the integrated avian sarcoma virus (ASV) proviral DNA and predicts a mechanism for recombination events that will lead to either the capture or the excision of the transforming gene of this virus. The latter is supported by our observation that there is an extensive homologous region on either side of the transforming gene that will allow site-specific deletion or integration to occur. Comparison of the clone derived from the src-specific 21S mRNA coding for the transforming gene product to that derived from the env-specific 21S mRNA coding for the envelope glycoprotein show that the common c region present at the 3' terminus of the ASV genome is 326 bases long. Within this c region are nucleotide sequences that may play key roles in the life cycle of this virus. These regulatory sequences include (i) probable promoter sites for the initiation of transcription, (ii) a polyadenylation signal, and (iii) a sequence that is complementary to the 3' termini of both the env and the src regions, which will allow the generation of transformation-defective deletions.

Synthesis of new proteins associated with the induction of interferon in human fibroblast cells

The relative amounts of translatable cellular mRNAs and newly synthesized cellular proteins were examined in poly(I) x poly(C)-induced human fibroblast cells early during induction. At this time interferon and interferon mRNA synthesis are maximal and cells have not acquired their antiviral thesis are maximal and cells have not acquired their antiviral state. Translation of the mRNA from poly(I) x poly(C)-induced cells in a wheat germ cell-free system led to the synthesis of a [35S]methionine-labeled 22,000-dalton protein that is precipitated by antiserum to highly purified human fibroblast interferon. The synthesis of this protein was detected only with the mRNA preparations that, when translated in Xenopus oocytes, coded for the synthesis of biologically active human interferon. Two-dimensional gel analysis of the [35S]methionine-labeled polypeptides translated from the total mRNA of the induced and uninduced cells revealed the presence of 23 new proteins that were translated from mRNAs of the induced cells but not from the mRNAs of the controls. These polypeptides ranged from 15,000 to 70,000 daltons. Thirteen of these proteins were detected in induced cells labeled with [35S]methionine. It is concluded that, in human fibroblasts, poly(I) x poly(C) induces, in addition to interferon, the synthesis of a variety of "interferon-associated" proteins.

Myoblast stored myosin heavy chain transcripts are precursors to the myotube polysomal myosin heavy chain mRNAs

Radioactively labeled myosin heavy chain messenger ribonucleic acid (MHC mRNA) synthesized during the pre-fusion stage of chick embryo breast muscle cell culture is transferred from messenger ribonucleic acid proteins (mRNPs) to the polysomal MHC mRNA during the period of rapid increase in the rate of MHC synthesis (mid-to late-fusion). This transfer constitutes a major contribution to the rate of incorporation of 3H-labeled transcripts into polysomal MHC mRNA at this time. As the increase in the rate of MHC synthesis levels off (late-to post-fusion) the contribution to the rate of incorporation of 3H-labeled transcripts into polysomal MHC mRNA from newly synthesized transcripts increases until it becomes predominant. In vivo, the level of MHC mRNP increases during early stages of embryonic development and then decreases when MHC synthesis and the level of polysomal MHC mRNA has been shown to increase.

CHO mutants resistant to colchicine, colcemid or griseofulvin have an altered beta-tubulin

Single-step mutants of Chinese hamster ovary (CHO) cells have been isolated which are resistant to killing by the anti-mitotic drugs colchicine, colcemid or griseofulvin. Two-dimensional gel analysis showed that two mutants resistant to griseofulvin, one resistant to colcemid and one resistant to colchicine carry an alteration in the beta-tubulin subunit. Most of the remaining isolates are believed to be permeability mutants on the basis of their cross resistance to drugs which do not interefere with microtubular polymerization or function (Ling and Thompson, 1974; Bech-Hansen, Till and Ling, 1976). A reduced amount of the wild-type beta-tubulin protein remained in each of the beta-tubulin mutants, but a beta-tubulin protein with a more basic isoelectric point also appeared. Messenger RNAs coding for both wild-type and variant beta-tubulins were found in at least one mutant as assayed by in vitro translation in a reticulocyte lysate. This indicates that the altered tubulin does not arise as the result of a posttranslational modification.

Extraction and translation of collagen mRNA from fetal calf skin

RNA was extracted from fetal calf skin by two different procedures, using phenol or guanidine hydrochloride. Poly(A)-rich RNA was separated by oligo(dT)-cellulose affinity chromatography and was further fractionated by sucrose density gradient centrifugation. When translated in an optimized wheat germ extract cell-free system, unfractionated guanidine-hydrochloride-extracted poly(A)-rich RNA directed the synthesis of two collagenase-sensitive protein bands, while phenol-extracted poly(A)-rich RNA with a sedimentation coefficient higher than 25 S was the only fraction to direct the same synthesis. On the basis of their electrophoretic mobility on a sodium dodecylsulfate/urea/polyacrylamide gel, these proteins were identified with procollagen alpha 1(I) and procollagen alpha 2. Inhibition of translation by phenol-extracted poly(A)-rich RNA with a sedimentation coefficient lower than 25 S was also observed. Guanidine-hydrochloride-extracted poly(A)-rich RNA from fetal skin directed the synthesis of three distinct collagenase-sensitive proteins in the micrococcal-nuclease-digested rabbit reticulocyte cell-free system; these seemed to correspond to procollagen alpha 1(I), procollagen alpha 2 and procollagen alpha 1 (III).

Changes in keratin gene expression during terminal differentiation of the keratinocyte

Cells of the inner layers of the epidermis contain small keratins (46-58K), whereas the cells of the outer layers contain large keratins (63-67K) in addition to small ones. The changes in keratin composition that take place within each cell during the course of its terminal differentiation result largely from changes in synthesis. Cultured epidermal cells resemble cells of the inner layers of the epidermis in synthesizing only small keratins. The cultured cells possess translatable mRNA only for small keratins, whereas mRNA extracted from whole epidermis can be translated into both large and small keratins. As no synthesis takes place in the outermost layer of the epidermis (stratum corneum), the keratins of this layer must be synthesized earlier, but in some cases they then become smaller: this presumably occurs by post-translational processing of the molecules during the final stages of differentiation. Stratified squamous epithelia of internal organs do not form a typical stratum corneum and do not make the large keratins characteristic of epidermis. Their keratins are also different from those of cultured keratinocytes, implying that they have embarked on an alternate route of terminal keratin synthesis.

Unusual features in the nucleotide sequence of a cDNA clone derived from the common region of avian sarcoma virus messenger RNA

We have constructed a recombinant plasmid containing a 700-base pair (bp) cDNA copy of the common region present at the 3' end of Schmidt-Ruppin avian sarcoma virus (ASV) 21S mRNA. The cDNA was inserted into plasmid pBR322 at the Pst I site by the G-C tailing method. A restriction map of the cloned insert from a recombinant plasmid pSRI indicates that it corresponds to the 3' end of the ASV genome. R-loop analysis with ASV genomic RNA indicates that the insert is colinear with the ASV genome over most of its length. The sequence of 331 bp at the 3' end of the DNA insert was determined and shows that the insert contains extra sequences not found at the 3' end of ASV genomic RNA. Following the terminally redundant sequence of 20 bp that has been found at the extreme 3' end of genomic RNA is a sequence of 79 bp that is almost identical to that located immediately next to the 20-bp repeat at the 5' end of ASV genomic RNA. This is followed by 18 bp of unique sequence, possibly of host origin. The structure of the clone suggests that ASV mRNA may differ from genomic RNA at its 3' end and that 21S mRNA is transcribed from integrated ASV DNA and contains at its 3' end sequences derived both from the 5' end of the ASV genome and from host DNA adjacent to the site of integration. The presence of termination codons in all three reading frames suggests that the common region probably does not contain coding sequences. However, the presence of sequences that resemble probable promoter sites supports the possibility that this region may be involved in the regulation of transcription.

Expression of the zygotic genome in blastoderm stage embryos ofDrosophila: Analysis of a specific protein

The synthesis of a protein which has been detected in blastoderm cells but not in pole cells (Gutzeit and Gehring 1979) has been studied further by means of two-dimensional gel electrophoresis. This protein could not be detected at the nuclear multiplication stage. The protein is translated from mRNA which is transcribed at the blastoderm stage since it is not synthesized in detectable amounts when embryos are injected with α-amanitin prior to the blastoderm stage. Also the protein could not be detected when RNA from freshly laid eggs was translated in vitro. Embryos from females which are homozygous for the mutationmat (3) 1 form pole cells but no blastoderm cells (Rice and Garen 1975). Thesemat (3) 1 embryos, as we will call them in this report, express the protein if aged for a period of time sufficient for completion of blastoderm cell formation in control wild-type embryos.mat (3) 1 embryos and embryos injected with α-amanitin show the same syndrome of visible developmental anomalies; however, the studied protein could only be detected inmat (3) 1 embryos but not in α-amanitin injected embryos.

Cloning of synthetic deoxyribonucleic acid that codes for embryonic cardiac myosin light-chain polypeptide

Double-stranded complementary deoxyribonucleic acid (cDNA) transcribed in vitro from a partially pure myosin light-chain messenger ribonucleic acid (mRNA) of the chick embryonic heart was cloned in Escherichia coli strain chi 1776 by using the HindIII cleavage site in the plasmid pBR322. The insertion of essentially full length DNA was achieved by repeated selection of large-size cDNA transcripts. Of the 12 transformants that contained large-size DNA inserts, the clone pML10 insert was 950 base pairs in length, almost the same size as myosin light-chain mRNA (980 nucleosides). The clone pML10 was identified by hybridization with a highly pure cDNA probe and by hybrid-arrested translation assay. pML10 was further characterized by partial restriction enzyme mapping. The availability of a cloned DNA probe for myosin light-chain facilitates the analysis of the mechanism underlying the induction of cardiac muscle specific gene transcription in presumptive heart-forming cells of the chick blastoderm.

[The regulation of protein synthesis in heart muscle. Biochemical data, stimulative and inhibitory factors and their clinical significance (author's transl)]

The regulation of protein synthesis in heart muscle has been investigated by many authors under both normal and pathological conditions. This review summarizes the evidence for the dependence of normal heart protein synthesis from normal serum levels of insulin, amino acids, fatty acids and glucose. A decreased serum concentration of these substances causes an inhibition of heart muscle protein synthesis by 30--60%. Various drugs and other chemical lead to similar impairments of heat muscle protein synthesis. The resulting imbalance between synthesis and degradation of myocardial proteins with their half-times of 5--12 days gradually leads to a decrease in their myocellular concentration with a consequent impairment of myocardial function. Finally, the biochemial sequences are described which represent the important pathogenetic mechanisms in the development of heart muscle hypertrophy and in the adriamycin-induced cardiomyopathy.

Manipulation of myogenesis in vitro: reversible inhibition by DMSO

A system has been developed for the detailed analysis of the transition from proliferative myoblast to differentiated muscle cell. Dimethylsulfoxide (DMSO) prevents the terminal differentiation of L8 myoblasts in vitro, and its effect is reversible. DMSO (2%) inhibits the fusion of myoblasts to form multinucleate myotubes, the normal increases in activity of creatine phosphokinase (CPK) and acetylcholinesterase, and the synthesis of alpha-actin and acetylcholine receptor protein. Upon removal of DMSO from the medium, a lag precedes the onset of differentiation. The potential to inhibit muscle differentiation reversibly is not specific to DMSO, but is shared by a number of compounds, including dimethylformamide, hexamethylbisacetamide and butyric acid, all potent inducers of gene expression in Friend erythroleukemia cells. L8 cells routinely cease DNA synthesis and initiate fusion and muscle protein synthesis once they are confluent. In the presence of DMSO, however, nearly all cells continue DNA synthesis, even several days after reaching confluence. Protein synthetic patterns of DMSO-inhibited cells are almost indistinguishable from those of untreated myoblasts and distinct from differentiated myotubes. It appears that cells exposed to DMSO are locked indefinitely in a proliferative myoblast stage of development and are unable to enter the Go phase of the cell cycle necessary for initiation of differentiation. DMSO coordinately inhibits all the differentiative parameters measured. In contrast, cytochalasin B uncouples normally linked differentiative events so that fusion is inhibited while muscle-specific protein synthesis proceeds. DMSO has similar effects on both cytochalasin B-treated and fusing control cultures, suggesting that its primary effect is exerted not at the level of fusion but earlier in the differentiative time-table. Once fusion and the synthesis of muscle-specific proteins are well under way, the addition of DMSO is ineffective and differentiation continues in its presence. The potential to manipulate muscle gene expression in vitro makes this system particularly useful for the detailed analysis of the processes involved in the transition to the differentiated state and for determining the linkage of developmental events.

Cell-free translation of mammalian myosin heavy-chain messenger ribonucleic acid from growing and fused-L6E9 myoblasts

An mRNA-dependent reticulocyte cell-free protein synthesizing system very efficient in the translation of myosin heavy-chain mRNA from a rat myogenic cell line is described. This system exhibits a high degree of fidelity with regard to the spectrum and relative proportion of the different proteins synthesized from a sample of cytoplasmic RNA as compared to the proteins synthesized in vivo by the cells from which the RNA is prepared. The main feature of this system is the use of a K+ and Cl- concentration similar to those of the reticulocyte cytoplasm. Using this system, myosin heavy chain, identified by low-salt precipitation, electrophoretic mobility, and partial peptide analysis, represents 17% of the total protein synthesis when cytoplasmic RNA from well-fused L6E9 cells is used. Furthermore, when RNA preparations from growing myoblasts, that when analyzed in other cell-free translational systems seem not to contain any myosin heavy-chain mRNA, are tested in the system reported here, they are proven to contain high amounts of translatable myosin heavy-chain mRNA.

Sub-cellular distribution of the cytoplasmic myosin heavy chain mRNA during myogenesis

In the light of earlier work [1] which demonstrated the presence of a large number of myosin heavy chain (MHC) transcripts in chick myoblasts prior to cell fusion and the burst of MHC synthesis it was of great interest to determine the subcellular localization of the still inactive transcripts. It has been determined in differentiating muscle cells in culture. Two populations of cells were examined -- monucleated myoblasts just prior to cell fusion and myotubes where at least 80% of the cells were fused. Utilizing a myosin complementary DNA (cDNA) probe [2] it is observed that just prior to cell fusion, when the "burst" of myosin synthesis has not yet occurred, the vast majority of cytoplasmic myosin mRNA transcripts are found in a stored messenger RNA protein complex with a minimal amount found in the heavy polysome fraction. In differentiated myotube cultures, when myosin synthesis is progressing at a high rate, the reverse is found, i.e, the amount of stored myosin messenger RNA (mRNA) is minimal while the largest amount of myosin mRNA transcripts are localized in the polysome fraction. The number of total cytoplasmic myosin transcripts is found to decrease after cell fusion at a time when myosin synthesis is maximal suggesting that the efficiency of translation of myosin mRNA increases during terminal differentiation.