The complete sequence of the mouse skeletal alpha-actin gene reveals several conserved and inverted repeat sequences outside of the protein-coding region

DNA methylation profile of the mouse skeletal alpha-actin promoter during development and differentiation

Genomic levels of DNA methylation undergo widespread alterations in early embryonic development. However, changes in embryonic methylation have proven difficult to study at the level of single-copy genes due to the small amount of tissue available for assay. This study provides the first detailed analysis of the methylation state of a tissue-specific gene through early development and differentiation. Using bisulfite sequencing, we mapped the methylation profile of the tissue-specific mouse skeletal alpha-actin promoter at all stages of development, from gametes to postimplantation embryos. We show that the alpha-actin promoter, which is fully methylated in the sperm and essentially unmethylated in the oocyte, undergoes a general demethylation from morula to blastocyst stages, although the blastula is not completely demethylated. Remethylation of the alpha-actin promoter occurs after implantation in a stochastic pattern, with some molecules being extensively methylated and others sparsely methylated. Moreover, we demonstrate that tissue-specific expression of the skeletal alpha-actin gene in the adult mouse does not correlate with the methylation state of the promoter, as we find a similar low level of methylation in both expressing and one of the two nonexpressing tissues tested. However, a subset of CpG sites within the skeletal alpha-actin promoter are preferentially methylated in liver, a nonexpressing tissue.

Tissue-restricted expression of the cardiac alpha-myosin heavy chain gene is controlled by a downstream repressor element containing a palindrome of two ets-binding sites

The expression of the alpha-myosin heavy chain (MHC) gene is restricted primarily to cardiac myocytes. To date, several positive regulatory elements and their binding factors involved in alpha-MHC gene regulation have been identified; however, the mechanism restricting the expression of this gene to cardiac myocytes has yet to be elucidated. In this study, we have identified by using sequential deletion mutants of the rat cardiac alpha-MHC gene a 30-bp purine-rich negative regulatory (PNR) element located in the first intronic region that appeared to be essential for the tissue-specific expression of the alpha-MHC gene. Removal of this element alone elevated (20- to 30-fold) the expression of the alpha-MHC gene in cardiac myocyte cultures and in heart muscle directly injected with plasmid DNA. Surprisingly, this deletion also allowed a significant expression of the alpha-MHC gene in HeLa and other nonmuscle cells, where it is normally inactive. The PNR element required upstream sequences of the alpha-MHC gene for negative gene regulation. By DNase I footprint analysis of the PNR element, a palindrome of two high-affinity Ets-binding sites (CTTCCCTGGAAG) was identified. Furthermore, by analyses of site-specific base-pair mutation, mobility gel shift competition, and UV cross-linking, two different Ets-like proteins from cardiac and HeLa cell nuclear extracts were found to bind to the PNR motif. Moreover, the activity of the PNR-binding factor was found to be increased two- to threefold in adult rat hearts subjected to pressure overload hypertrophy, where the alpha-MHC gene is usually suppressed. These data demonstrate that the PNR element plays a dual role, both downregulating the expression of the alpha-MHC gene in cardiac myocytes and silencing the muscle gene activity in nonmuscle cells. Similar palindromic Ets-binding motifs are found conserved in the alpha-MHC genes from different species and in other cardiac myocyte-restricted genes. These results are the first to reveal a role of the Ets class of proteins in controlling the tissue-specific expression of a cardiac muscle gene.

An alternative promoter in the mouse major histocompatibility complex class II I-Abeta gene: implications for the origin of CpG islands

Nonmethylated CpG islands are generally located at the 5' ends of genes, but a CpG island in the mouse major histocompatibility complex class II I-Abeta gene is remote from the promoter and covers exon 2. We have found that this CpG island includes a novel intronic promoter that is active in embryonic and germ cells. The resulting transcript potentially encodes a severely truncated protein which would lack the signal peptide and external beta1 domains. The functional significance of the internal CpG island may be to facilitate gene conversion, thereby sustaining the high level of polymorphism seen at exon 2. Deletions of the I-Abeta CpG island promoter reduce transcription and frequently lead to methylation of the CpG island in a transgenic mouse assay. These and other results support the idea that all CpG islands arise at promoters that are active in early embryonic cells.

Electrical activity-dependent regulation of the acetylcholine receptor delta-subunit gene, MyoD, and myogenin in primary myotubes

Expression of the skeletal muscle acetylcholine receptor (AChR) is regulated by nerve-evoked muscle activity. Studies using transgenic mice have shown that this regulation is controlled largely by transcriptional mechanisms because responsiveness to electrical activity can be conferred by transgenes containing cis-acting sequences from the AChR subunit genes. The lack of a convenient muscle cell culture system for studying electrical activity-dependent gene regulation, however, has made it difficult to identify the important cis-acting sequences and to characterize an electrical activity-dependent signaling pathway. We developed a muscle culture system to study the mechanisms of electrical activity-dependent gene expression. Gene fusions between the murine AChR delta-subunit gene and the human growth hormone gene were transfected into primary myoblasts, and the amount of growth hormone secreted into the culture medium from either spontaneously electrically active or inactive myotube cultures was measured. We show that 181 bp of 5'-flanking DNA from the AChR delta-subunit gene are sufficient to confer electrical activity-dependent gene expression. In addition, we show that the rate of AChR delta-subunit gene expression differs among individual nuclei in a single myotube but that highly expressing nuclei are not necessarily colocalized with AChR clusters. We also show that expression of MyoD and myogenin are regulated by electrical activity in primary myotube cultures and that all nuclei within a myotube express similar levels of MyoD and similar levels of myogenin.

Linkage of Agt and Actsk-1 to distal mouse chromosome 8 loci: a new conserved linkage

Angiotensinogen is an alpha 2-globulin involved in the maintenance of blood pressure and electrolyte balance. We have refined the position of the mouse angiotensinogen locus (Agt) on Chromosome (Chr) 8 and have also confirmed the assignment of the human angiotensinogen locus (AGT) to Chr 1. The segregation of several restriction fragment length variants (RFLVs) was followed in two interspecific backcross sets and in four recombinant inbred (RI) mouse sets. Analysis of the segregation patterns closely linked Agt to Aprt and Emv-2, which places the angiotensinogen locus on the distal end of mouse Chr 8. Additionally, a literature search has revealed that the strain distribution pattern (SDP) for the mouse skeletal alpha-actin locus 1 (Actsk-1, previously Acta1, Acta, or Acts) is nearly identical to the SDP for Agt in two RI sets. On the basis of this information we were able to reassign Actsk-1 to mouse Chr 8. By screening a panel of human-mouse somatic cell hybrids, we confirmed that the human angiotensinogen locus lies on Chr 1. This information describes a new region of conserved linkage homology between mouse Chr 8 and human Chr 1. It also defines the end of a large region of conserved linkage homology between mouse Chr 8 and human Chr 16.

Re-localization of Actsk-1 to mouse chromosome 8, a new region of homology with human chromosome 1

We present here the genetic mapping of the alpha-skeletal actin locus (Actsk-1) on mouse Chromosome (Chr) 8, on the basis of the PCR analysis of a microsatellite in an interspecific backcross. Linkage and genetic distances were established for four loci by analysis of 192 (or 222) meiotic events and indicated the following gene order: (centromere)-Es-1-11.7 cM-Tat-8.3 cM-Actsk-1-0.5 cM-Aprt. Mapping of ACTSK to human Chr 1 and of TAT and APRT to human Chr 16 demonstrates the existence of a new short region of homology between mouse Chr 8 and human Chr 1. Intermingling on this scale between human and mouse chromosomal homologies that occurred during evolution creates disorders in comparative linkage studies.

Identification of single-stranded-DNA-binding proteins that interact with muscle gene elements

A sequence-specific DNA-binding protein from skeletal-muscle extracts that binds to probes of three muscle gene DNA elements is identified. This protein, referred to as muscle factor 3, forms the predominant nucleoprotein complex with the MCAT gene sequence motif in an electrophoretic mobility shift assay. This protein also binds to the skeletal actin muscle regulatory element, which contains the conserved CArG motif, and to a creatine kinase enhancer probe, which contains the E-box motif, a MyoD-binding site. Muscle factor 3 has a potent sequence-specific, single-stranded-DNA-binding activity. The specificity of this interaction was demonstrated by sequence-specific competition and by mutations that diminished or eliminated detectable complex formation. MyoD, a myogenic determination factor that is distinct from muscle factor 3, also bound to single-stranded-DNA probes in a sequence-specific manner, but other transcription factors did not. Multiple copies of the MCAT motif activated the expression of a heterologous promoter, and a mutation that eliminated expression was correlated with diminished factor binding. Muscle factor 3 and MyoD may be members of a class of DNA-binding proteins that modulate gene expression by their abilities to recognize DNA with unusual secondary structure in addition to specific sequence.

Identification of single-stranded-DNA-binding proteins that interact with muscle gene elements

A sequence-specific DNA-binding protein from skeletal-muscle extracts that binds to probes of three muscle gene DNA elements is identified. This protein, referred to as muscle factor 3, forms the predominant nucleoprotein complex with the MCAT gene sequence motif in an electrophoretic mobility shift assay. This protein also binds to the skeletal actin muscle regulatory element, which contains the conserved CArG motif, and to a creatine kinase enhancer probe, which contains the E-box motif, a MyoD-binding site. Muscle factor 3 has a potent sequence-specific, single-stranded-DNA-binding activity. The specificity of this interaction was demonstrated by sequence-specific competition and by mutations that diminished or eliminated detectable complex formation. MyoD, a myogenic determination factor that is distinct from muscle factor 3, also bound to single-stranded-DNA probes in a sequence-specific manner, but other transcription factors did not. Multiple copies of the MCAT motif activated the expression of a heterologous promoter, and a mutation that eliminated expression was correlated with diminished factor binding. Muscle factor 3 and MyoD may be members of a class of DNA-binding proteins that modulate gene expression by their abilities to recognize DNA with unusual secondary structure in addition to specific sequence.

Common factor 1 is a transcriptional activator which binds in the c-myc promoter, the skeletal alpha-actin promoter, and the immunoglobulin heavy-chain enhancer

Ubiquitously expressed transcription factors play an integral role in establishing and regulating patterns of gene transcription. Common factor 1 (CF1) is a ubiquitously expressed DNA-binding protein previously identified in our laboratory. We show here that CF1 recognizes sites in several diverse transcription elements, and we demonstrate the ability of the c-myc CF1 site to activate transcription of a basal promoter in both B cells and fibroblasts.

Adrenergic regulation of the skeletal alpha-actin gene promoter during myocardial cell hypertrophy

The skeletal alpha-actin gene is expressed in fetal rat heart and is induced during norepinephrine (NE)-stimulated hypertrophy in cultures of neonatal rat cardiac myocytes. Here we report that NE positively regulates the human skeletal alpha-actin gene promoter in transiently transfected neonatal rat cardiac myocytes. NE increased expression from the full-length promoter by 2.4-fold. A DNA region required for NE responsiveness but not for tissue-specific expression was located between base pair -2000 and base pair -1300. Distinct regions required for cardiac myocyte expression were located between -1300 to -710 and -153 to -87. None of these elements separately conferred tissue specificity or adrenergic responsiveness on a heterologous promoter, although the intact promoter from -2000 to -36 conferred both when cloned in its correct position and orientation. Additional elements in the basal promoter (-87 to +187) were required for maximal NE responsiveness. The NE induction was mediated by the beta-adrenergic receptor in high-density cultures (3-4 x 10(6) cells per 60-mm dish), as was induction of hypertrophy, contractility, and endogenous skeletal alpha-actin gene expression. The beta-adrenergic agonist isoproterenol was as potent as NE in inducing expression. Furthermore, beta-adrenergic antagonists inhibited the effects on skeletal alpha-actin gene expression but alpha 1-adrenergic antagonists did not. The alpha 1-adrenergic system was intact in these high-density cultures, since the effects of NE on the expression of another contractile protein gene, alpha-myosin heavy chain, were blocked by alpha 1- but not by beta-adrenergic antagonists. In these high-density cultures, cell contact and intermyocardiocyte bridging were prevalent. When cardiac myocytes were plated at a low density, minimizing cell contact, NE induction of skeletal alpha-actin gene expression and hypertrophy was mediated by the alpha 1-adrenoceptor. Factors related to cell communication may influence the pathways mediating NE-regulated gene transcription during cardiac myocyte hypertrophy.

Colony-stimulating factor 1 expression is down-regulated during the adipocyte differentiation of H-1/A marrow stromal cells and induced by cachectin/tumor necrosis factor

We isolated clonal sublines of the established mouse marrow stromal cell line, H-1. These clonal sublines underwent differentiation into adipocytes in various degrees. One subline, H-1/A, underwent adipocyte differentiation after confluence, while another subline, H-1/D, did not differentiate. In H-1/A cells, the 4.5- and 2.5-kb major mRNA species of colony-stimulating factor 1 (CSF-1) were expressed before differentiation and were down-regulated at a posttranscriptional level during the differentiation of H-1/A cells. The down-regulation of the CSF-1 gene was not a result of arrested cellular growth, because no down-regulation was detected in the nondifferentiating sister line, H-1/D. This down-regulation appeared to be an early event in differentiation. Cachectin/tumor necrosis factor transiently induced the expression of CSF-1 and inhibited the differentiation of H-1/A cells into adipocytes. This induced expression of CSF-1 was due to an increased rate of transcription.

Common factor 1 is a transcriptional activator which binds in the c-myc promoter, the skeletal alpha-actin promoter, and the immunoglobulin heavy-chain enhancer

Ubiquitously expressed transcription factors play an integral role in establishing and regulating patterns of gene transcription. Common factor 1 (CF1) is a ubiquitously expressed DNA-binding protein previously identified in our laboratory. We show here that CF1 recognizes sites in several diverse transcription elements, and we demonstrate the ability of the c-myc CF1 site to activate transcription of a basal promoter in both B cells and fibroblasts.

Colony-stimulating factor 1 expression is down-regulated during the adipocyte differentiation of H-1/A marrow stromal cells and induced by cachectin/tumor necrosis factor

We isolated clonal sublines of the established mouse marrow stromal cell line, H-1. These clonal sublines underwent differentiation into adipocytes in various degrees. One subline, H-1/A, underwent adipocyte differentiation after confluence, while another subline, H-1/D, did not differentiate. In H-1/A cells, the 4.5- and 2.5-kb major mRNA species of colony-stimulating factor 1 (CSF-1) were expressed before differentiation and were down-regulated at a posttranscriptional level during the differentiation of H-1/A cells. The down-regulation of the CSF-1 gene was not a result of arrested cellular growth, because no down-regulation was detected in the nondifferentiating sister line, H-1/D. This down-regulation appeared to be an early event in differentiation. Cachectin/tumor necrosis factor transiently induced the expression of CSF-1 and inhibited the differentiation of H-1/A cells into adipocytes. This induced expression of CSF-1 was due to an increased rate of transcription.

Structure and expression of a single actin gene in Volvox carteri

Southern blot analysis of Volvox carteri DNA indicated the presence of a single actin gene; the nucleotide sequence of that gene is reported here. In comparison with plant animal and fungal actins, the derived primary structure of 377 amino acids is highly conserved yielding similarity values of 79% to 94% (including non-identical conservative exchanges). In contrast, the intron structure of the gene is highly unusual: in addition to one intron in the 5' untranslated region (ten nucleotides upstream of the initiator ATG), it has eight introns in the coding region, only three of which are in locations where introns have previously been reported. Transcription starts 26 nucleotides downstream of the putative TATA box and 70 nucleotides downstream of a conspicuous CCAAT motif. A potential polyadenylation signal, TGTAA, is located 366 nucleotides downstream of the terminator TAA. Northern hybridization indicates that the actin gene is transcribed throughout the Volvox life cycle with only a slight depression during the release of juveniles from mother spheroids. This pattern of gene expression suggests that actin may assume various functional roles in the differentiation and growth of Volvox.

Effect of thyroid hormone on the accumulation of mRNA for skeletal and cardiac alpha-actin in hearts from normal and hypophysectomized rats

Skeletal alpha-actin gene products are coexpressed with cardiac alpha-actins in cardiac tissue of adult humans, cows, and pigs; in prenatal rats; and during hypertrophy due either to increased hemodynamic load or the administration of alpha-adrenergic agonists. Because there is preferential synthesis of the beta-myosin heavy chain in each case, it has been suggested that the synthesis of skeletal alpha-actin in cardiac tissue is linked to that of beta-myosin heavy chain. To test this hypothesis, thyroid hormone, which causes cardiac hypertrophy with preferential synthesis of alpha-myosin heavy chain, was administered to normal and hypophysectomized rats. Animals were sacrificed from 2 to 24 hr after the injection of either 1 or 5 micrograms of hormone per 10 g of body weight. The relative amount of mRNA for skeletal and cardiac alpha-actin was measured by using the technique of primer extension. Thyroid hormone caused a rapid increase in the amount of skeletal alpha-actin mRNA relative to controls, more than 7 times in hearts from normal animals and 15 times in hearts from hypophysectomized animals. A small increase in cardiac alpha-actin mRNA also occurred. The rapid increase in transcripts for skeletal alpha-actin under conditions where the isoform of myosin heavy chain that is being synthesized is primarily alpha demonstrates independent patterns of activation of the actin and myosin heavy chain multigene families during cardiac growth in mammals.

A combination of closely associated positive and negative cis-acting promoter elements regulates transcription of the skeletal alpha-actin gene

The chicken skeletal alpha-actin gene promoter region provides at least a 75-fold-greater transcriptional activity in muscle cells than in fibroblasts. The cis-acting sequences required for cell type-restricted expression within this 200-base-pair (bp) region were elucidated by chloramphenicol acetyltransferase assays of site-directed Bg/II linker-scanning mutations transiently transfected into primary cultures. Four positive cis-acting elements were identified and are required for efficient transcriptional activity in myogenic cells. These elements, conserved across vertebrate evolution, include the ATAAAA box (-24 bp), paired CCAAT-box-associated repeats (CBARs; at -83 bp and -127 bp), and the upstream T+A-rich regulatory sequence (at -176 bp). Basal transcriptional activity in fibroblasts was not as dependent on the upstream CBAR or regions of the upstream T+A-rich regulatory sequence. Transfection experiments provided evidence that positive regulatory factors required for alpha-actin expression in fibroblasts are limiting. In addition, negative cis-acting elements were detected and found closely associated with the G+C-rich sequences that surround the paired CBARs. Negative elements may have a role in restricting developmentally timed expression in myoblasts and appear to inhibit promoter activity in nonmyogenic cells. Cell type-specific expression of the skeletal alpha-actin gene promoter is regulated by combinatorial and possibly competitive interactions between multiple positive and negative cis-acting elements.

A combination of closely associated positive and negative cis-acting promoter elements regulates transcription of the skeletal alpha-actin gene

The chicken skeletal alpha-actin gene promoter region provides at least a 75-fold-greater transcriptional activity in muscle cells than in fibroblasts. The cis-acting sequences required for cell type-restricted expression within this 200-base-pair (bp) region were elucidated by chloramphenicol acetyltransferase assays of site-directed Bg/II linker-scanning mutations transiently transfected into primary cultures. Four positive cis-acting elements were identified and are required for efficient transcriptional activity in myogenic cells. These elements, conserved across vertebrate evolution, include the ATAAAA box (-24 bp), paired CCAAT-box-associated repeats (CBARs; at -83 bp and -127 bp), and the upstream T+A-rich regulatory sequence (at -176 bp). Basal transcriptional activity in fibroblasts was not as dependent on the upstream CBAR or regions of the upstream T+A-rich regulatory sequence. Transfection experiments provided evidence that positive regulatory factors required for alpha-actin expression in fibroblasts are limiting. In addition, negative cis-acting elements were detected and found closely associated with the G+C-rich sequences that surround the paired CBARs. Negative elements may have a role in restricting developmentally timed expression in myoblasts and appear to inhibit promoter activity in nonmyogenic cells. Cell type-specific expression of the skeletal alpha-actin gene promoter is regulated by combinatorial and possibly competitive interactions between multiple positive and negative cis-acting elements.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum utilizes alternate promoters and splicing for the synthesis of multiple mRNAs

The cyclic nucleotide phosphodiesterase (phosphodiesterase) gene plays essential roles in the development of Dictyostelium discoideum during cellular aggregation and postaggregation morphogenesis. Genomic clones spanning the gene were isolated and used to determine the sequence and structure of the phosphodiesterase gene. We found an unusually complex organization for a gene of D. discoideum. Two transcripts of 2.4 and 1.9 kilobases (kb) were synthesized from start sites separated by 1.1 kb. A developmentally regulated promoter was utilized for the 2.4-kb mRNA, and a constitutive promoter regulated synthesis of the 1.9-kb transcript. The gene was found to be divided into four exons that are alternately spliced to give rise to the two mRNAs. The precursor of the 2.4-kb mRNA contained a 2.3-kb intron, whereas the precursor of the constitutive transcript was synthesized with a 1.7-kb intron. The two transcripts contained identical protein-coding regions and 400-nucleotide 3' untranslated sequences. The 2.4-kb developmentally regulated mRNA was distinguished by a long 5' untranslated leader of 666 nucleotides. The complex structure of the gene may allow multiple levels of control of the expression of the phosphodiesterase during development.

The chicken skeletal muscle alpha-actin promoter is tissue specific in transgenic mice

We have generated transgenic mouse lines that carry the promoter region of the chicken skeletal muscle alpha (alpha sk) actin gene linked to the bacterial chloramphenicol acetyltransferase (CAT) gene. In adult mice, the pattern of transgene expression resembled that of the endogenous alpha sk actin gene. In most of the transgenic lines, high levels of CAT activity were detected in striated muscle (skeletal and cardiac) but not in the other tissues tested. In striated muscle, transcription of the transgene was initiated at the normal transcriptional start site of the chicken alpha sk actin gene. The region from nucleotides -191 to +27 of the chicken alpha sk actin gene was sufficient to direct the expression of CAT in striated muscle of transgenic mice. These observations suggest that the mechanism of tissue-specific actin gene expression is well conserved in higher vertebrate species.

The cyclic nucleotide phosphodiesterase gene of Dictyostelium discoideum utilizes alternate promoters and splicing for the synthesis of multiple mRNAs

The cyclic nucleotide phosphodiesterase (phosphodiesterase) gene plays essential roles in the development of Dictyostelium discoideum during cellular aggregation and postaggregation morphogenesis. Genomic clones spanning the gene were isolated and used to determine the sequence and structure of the phosphodiesterase gene. We found an unusually complex organization for a gene of D. discoideum. Two transcripts of 2.4 and 1.9 kilobases (kb) were synthesized from start sites separated by 1.1 kb. A developmentally regulated promoter was utilized for the 2.4-kb mRNA, and a constitutive promoter regulated synthesis of the 1.9-kb transcript. The gene was found to be divided into four exons that are alternately spliced to give rise to the two mRNAs. The precursor of the 2.4-kb mRNA contained a 2.3-kb intron, whereas the precursor of the constitutive transcript was synthesized with a 1.7-kb intron. The two transcripts contained identical protein-coding regions and 400-nucleotide 3' untranslated sequences. The 2.4-kb developmentally regulated mRNA was distinguished by a long 5' untranslated leader of 666 nucleotides. The complex structure of the gene may allow multiple levels of control of the expression of the phosphodiesterase during development.

The chicken skeletal muscle alpha-actin promoter is tissue specific in transgenic mice

We have generated transgenic mouse lines that carry the promoter region of the chicken skeletal muscle alpha (alpha sk) actin gene linked to the bacterial chloramphenicol acetyltransferase (CAT) gene. In adult mice, the pattern of transgene expression resembled that of the endogenous alpha sk actin gene. In most of the transgenic lines, high levels of CAT activity were detected in striated muscle (skeletal and cardiac) but not in the other tissues tested. In striated muscle, transcription of the transgene was initiated at the normal transcriptional start site of the chicken alpha sk actin gene. The region from nucleotides -191 to +27 of the chicken alpha sk actin gene was sufficient to direct the expression of CAT in striated muscle of transgenic mice. These observations suggest that the mechanism of tissue-specific actin gene expression is well conserved in higher vertebrate species.

Identification and developmental expression of a smooth-muscle gamma-actin in postmeiotic male germ cells of mice

Mouse testis contains two size classes of actin mRNAs of 2.1 and 1.5 kilobases (kb). The 2.1-kb actin mRNA codes for cytoplasmic beta- and gamma-actin and is found throughout spermatogenesis, while the 1.5-kb actin mRNA is first detected in postmeiotic cells. Here we identify the testicular postmeiotic actin encoded by the 1.5-kb mRNA as a smooth-muscle gamma-actin (SMGA) and present its cDNA sequence. The amino acid sequence deduced from the postmeiotic actin cDNA sequence was nearly identical to that of a chicken gizzard SMGA, with one amino acid replacement at amino acid 359, where glutamine was substituted for proline. The nucleotide sequence of the untranslated region of the SMGA differed substantially from those of other isotypes of mammalian actins. By using the 3' untranslated region of the testicular SMGA, a highly specific probe was obtained. The 1.5-kb mRNA was detected in RNA from mouse aorta, small intestine, and uterus, but not in RNA isolated from mouse brain, heart, and spleen. Testicular SMGA mRNA was first detected and increased substantially in amount during spermiogenesis in the germ cells, in contrast to the decrease of the cytoplasmic beta- and gamma-actin mRNAs towards the end of spermatogenesis. Testicular SMGA mRNA was present in the polysome fractions, indicating that it was translated. These studies demonstrate the existence of an SMGA in male haploid germ cells. The implications of the existence of an SMGA in male germ cells are discussed.

Cross-binding of factors to functionally different promoter elements in c-fos and skeletal actin genes

A conserved 28-base-pair element in the skeletal actin promoter was sufficient to activate muscle-specific expression when placed upstream of a TATA element. This muscle regulatory element (MRE) is similar in structure to the serum response element (SRE), which is present in the promoters of the c-fos proto-oncogene and the nonmuscle actin genes. The SRE can function as a constitutive promoter element. Though the MRE and SRE differed in their tissue-specific expression properties, both elements bound to the same protein factors in vitro. These proteins are the serum response factor (SRF) and the muscle actin promoter factors 1 and 2 (MAPF1 and MAPF2). The SRF and MAPF proteins were resolved by chromatographic procedures, and they differed in their relative affinities for each element. The factors were further distinguished by their distinct, but overlapping, methylation interference footprint patterns on each element. These data indicate that the differences in tissue-specific expression may be due to a complex interaction of protein factors with these sequences.

The expression of sarcomeric muscle-specific contractile protein genes in BC3H1 cells: BC3H1 cells resemble skeletal myoblasts that are defective for commitment to terminal differentiation

The BC3H1 cell line has been used widely as a model for studying regulation of muscle-related proteins, such as the acetylcholine receptor, myokinase, creatine kinase, and actin. These cells, derived from a nitrosourea-induced mouse brain neoplasm, have some of the morphological characteristics of smooth muscle and have been shown to express the vascular smooth muscle isoform of alpha-actin. To provide further information about the contractile protein phenotype of BC3H1 and to gain additional insights into the possible tissue of origin of these cells, we have examined the expression of a battery of contractile protein genes. During rapid growth, subconfluent BC3H1 cells express the nonmuscle isoform of alpha-tropomyosin (alpha-Tm) and the nonsarcomeric isoforms of myosin heavy and light chains (MHCs and MLCs, respectively), but do not express troponin T(TnT). However, when BC3H1 cells differentiate in response to incubation in serum-deprived medium or upon approaching confluence, they express TnT as well as sarcomeric muscle isoforms of MHC, MLC 2 and 3, alpha-Tm, and alpha-actin. These results suggest that BC3H1 is a skeletal muscle cell line of ectodermal origin that is defective for commitment to terminal differentiation.

Cross-binding of factors to functionally different promoter elements in c-fos and skeletal actin genes

A conserved 28-base-pair element in the skeletal actin promoter was sufficient to activate muscle-specific expression when placed upstream of a TATA element. This muscle regulatory element (MRE) is similar in structure to the serum response element (SRE), which is present in the promoters of the c-fos proto-oncogene and the nonmuscle actin genes. The SRE can function as a constitutive promoter element. Though the MRE and SRE differed in their tissue-specific expression properties, both elements bound to the same protein factors in vitro. These proteins are the serum response factor (SRF) and the muscle actin promoter factors 1 and 2 (MAPF1 and MAPF2). The SRF and MAPF proteins were resolved by chromatographic procedures, and they differed in their relative affinities for each element. The factors were further distinguished by their distinct, but overlapping, methylation interference footprint patterns on each element. These data indicate that the differences in tissue-specific expression may be due to a complex interaction of protein factors with these sequences.

Identification and developmental expression of a smooth-muscle gamma-actin in postmeiotic male germ cells of mice

Mouse testis contains two size classes of actin mRNAs of 2.1 and 1.5 kilobases (kb). The 2.1-kb actin mRNA codes for cytoplasmic beta- and gamma-actin and is found throughout spermatogenesis, while the 1.5-kb actin mRNA is first detected in postmeiotic cells. Here we identify the testicular postmeiotic actin encoded by the 1.5-kb mRNA as a smooth-muscle gamma-actin (SMGA) and present its cDNA sequence. The amino acid sequence deduced from the postmeiotic actin cDNA sequence was nearly identical to that of a chicken gizzard SMGA, with one amino acid replacement at amino acid 359, where glutamine was substituted for proline. The nucleotide sequence of the untranslated region of the SMGA differed substantially from those of other isotypes of mammalian actins. By using the 3' untranslated region of the testicular SMGA, a highly specific probe was obtained. The 1.5-kb mRNA was detected in RNA from mouse aorta, small intestine, and uterus, but not in RNA isolated from mouse brain, heart, and spleen. Testicular SMGA mRNA was first detected and increased substantially in amount during spermiogenesis in the germ cells, in contrast to the decrease of the cytoplasmic beta- and gamma-actin mRNAs towards the end of spermatogenesis. Testicular SMGA mRNA was present in the polysome fractions, indicating that it was translated. These studies demonstrate the existence of an SMGA in male haploid germ cells. The implications of the existence of an SMGA in male germ cells are discussed.

The chicken skeletal alpha-actin gene promoter region exhibits partial dyad symmetry and a capacity to drive bidirectional transcription

The chicken skeletal alpha-actin gene promoter region (-202 to -12) provides myogenic transcriptional specificity. This promoter contains partial dyad symmetry about an axis at nucleotide -108 and in transfection experiments is capable of directing transcription in a bidirectional manner. At least three different transcription initiation start sites, oriented toward upstream sequences, were mapped 25 to 30 base pairs from TATA-like regions. The opposing transcriptional activity was potentiated upon the deletion of sequences proximal to the alpha-actin transcription start site. Thus, sequences which serve to position RNA polymerase for alpha-actin transcription may allow, in their absence, the selection of alternative and reverse-oriented start sites. Nuclear runoff transcription assays of embryonic muscle indicated that divergent transcription may occur in vivo but with rapid turnover of nuclear transcripts. Divergent transcriptional activity enabled us to define the 3' regulatory boundary of the skeletal alpha-actin promoter which retains a high level of myogenic transcriptional activity. The 3' regulatory border was detected when serial 3' deletions bisected the element (-91 CCAAA TATGG -82) which reduced transcriptional activity by 80%. Previously we showed that disruption of its upstream counterpart (-127 CCAAAGAAGG -136) resulted in about a 90% decrease in activity. These element pairs, which we describe as CCAAT box-associated repeats, are conserved in all sequenced vertebrate sarcomeric actin genes and may act in a cooperative manner to facilitate transcription in myogenic cells.

Isolation and characterization of the mouse acetylcholine receptor delta subunit gene: identification of a 148-bp cis-acting region that confers myotube-specific expression

We have isolated the gene encoding the delta subunit of the mouse skeletal muscle acetylcholine receptor (AChR) and have identified a 148-bp cis-acting region that controls cell type-specific and differentiation-dependent gene expression. The 5' flanking region of the delta subunit gene was fused to the protein-coding region of the chloramphenicol acetyltransferase (CAT) gene and gene fusions were transfected into C2 mouse skeletal muscle cells. Both transiently and stably transfected cells were assayed for CAT gene expression. Deletions from the 5' end of the mouse delta gene demonstrate that 148 bp of 5' flanking DNA is sufficient to confer cell type-specific and differentiation-dependent expression: CAT activity is present in transfected myotubes, but not in transfected 3T3 cells or 10T1/2 cells. Moreover, the level of CAT expression in myotubes transfected with constructs containing 148 bp of 5' flanking DNA from the delta subunit gene is identical to that in myotubes transfected with constructs containing 3.2 kb of 5' flanking DNA and similar to expression from the SV-40 early promoter. Increased CAT activity in myotubes is a result of an increased rate of transcription from the delta subunit promoter, since CAT RNA levels are also 35-fold more abundant in myotubes than myoblasts. In contrast, the SV-40 early promoter is similarly active in all cell types. Thus, 148 bp of 5' flanking DNA from the delta subunit gene contains all the information required for cell type-specific and differentiation-dependent expression of the AChR delta subunit.

Promoter analysis of myosin alkali light chain genes expressed in mouse striated muscle

There are three principal myosin alkali light chain (MLC) genes expressed in mouse striated muscle. The skeletal muscle gene MLC1F/MLC3F, the ventricular muscle/slow skeletal muscle gene MLC1V(MLC1S), and the atrial muscle/foetal striated muscle gene MLC1A(MLC1emb). MLC1V and MLC1A are expressed in both cardiac and skeletal muscle, and we show here that these genes use a single site of initiation of transcription, and therefore the same proximal promoter region, in both muscle types, and in myogenic cell lines in culture. We have previously shown that for the MLC1F/MLC3F gene, 1200bp of upstream sequence from the MLC1F promoter is sufficient to allow tissue specific and developmentally regulated expression. We have therefore isolated, characterised, and sequenced over 1200bp upstream of each of the three MLC genes in order to look for elements which may be involved in their regulation. Detailed comparison of their promoter sequences, as well as those of the cardiac and skeletal muscle alpha-actin genes, reveals a number of common elements. Among these is an "MLC-sequence" (CCTTTTATAG) common to all MLC genes, including those of chick and rat, and a "cardiac sequence" common to the mouse MLC1A, MLC1V and alpha-cardiac actin genes expressed in the heart.

The chicken skeletal alpha-actin gene promoter region exhibits partial dyad symmetry and a capacity to drive bidirectional transcription

The chicken skeletal alpha-actin gene promoter region (-202 to -12) provides myogenic transcriptional specificity. This promoter contains partial dyad symmetry about an axis at nucleotide -108 and in transfection experiments is capable of directing transcription in a bidirectional manner. At least three different transcription initiation start sites, oriented toward upstream sequences, were mapped 25 to 30 base pairs from TATA-like regions. The opposing transcriptional activity was potentiated upon the deletion of sequences proximal to the alpha-actin transcription start site. Thus, sequences which serve to position RNA polymerase for alpha-actin transcription may allow, in their absence, the selection of alternative and reverse-oriented start sites. Nuclear runoff transcription assays of embryonic muscle indicated that divergent transcription may occur in vivo but with rapid turnover of nuclear transcripts. Divergent transcriptional activity enabled us to define the 3' regulatory boundary of the skeletal alpha-actin promoter which retains a high level of myogenic transcriptional activity. The 3' regulatory border was detected when serial 3' deletions bisected the element (-91 CCAAA TATGG -82) which reduced transcriptional activity by 80%. Previously we showed that disruption of its upstream counterpart (-127 CCAAAGAAGG -136) resulted in about a 90% decrease in activity. These element pairs, which we describe as CCAAT box-associated repeats, are conserved in all sequenced vertebrate sarcomeric actin genes and may act in a cooperative manner to facilitate transcription in myogenic cells.

Identification of upstream and intragenic regulatory elements that confer cell-type-restricted and differentiation-specific expression on the muscle creatine kinase gene

Terminal differentiation of skeletal myoblasts is accompanied by induction of a series of tissue-specific gene products, which includes the muscle isoenzyme of creatine kinase (MCK). To begin to define the sequences and signals involved in MCK regulation in developing muscle cells, the mouse MCK gene has been isolated. Sequence analysis of 4,147 bases of DNA surrounding the transcription initiation site revealed several interesting structural features, some of which are common to other muscle-specific genes and to cellular and viral enhancers. To test for sequences required for regulated expression, a region upstream of the MCK gene from -4800 to +1 base pairs, relative to the transcription initiation site, was linked to the coding sequences of the bacterial chloramphenicol acetyltransferase (CAT) gene. Introduction of this MCK-CAT fusion gene into C2 muscle cells resulted in high-level expression of CAT activity in differentiated myotubes and no detectable expression in proliferating undifferentiated myoblasts or in nonmyogenic cell lines. Deletion mutagenesis of sequences between -4800 and the transcription start site showed that the region between -1351 and -1050 was sufficient to confer cell type-specific and developmentally regulated expression on the MCK promoter. This upstream regulatory element functioned independently of position, orientation, or distance from the promoter and therefore exhibited the properties of a classical enhancer. This upstream enhancer also was able to confer muscle-specific regulation on the simian virus 40 promoter, although it exhibited a 3- to 5-fold preference for its own promoter. In contrast to the cell type- and differentiation-specific expression of the upstream enhancer, the MCK promoter was able to function in myoblasts and myotubes and in nonmyogenic cell lines when combined with the simian virus 40 enhancer. An additional positive regulatory element was identified within the first intron of the MCK gene. Like the upstream enhancer, this intragenic element functioned independently of position, orientation, and distance with respect to the MCK promoter and was active in differentiated myotubes but not in myoblasts. These results demonstrate that expression of the MCK gene in developing muscle cells is controlled by complex interactions among multiple upstream and intragenic regulatory elements that are functional only in the appropriate cellular context.

Mouse brain CD4 transcripts encode only the COOH-terminal half of the protein

The T-cell surface glycoprotein CD4 is thought to function as a receptor for class II major histocompatibility complex molecules. Human CD4 is also the lymphoid cell receptor for human immunodeficiency virus, the causative agent of acquired immune deficiency syndrome. The observed infection of the central nervous system in acquired immune deficiency syndrome patients raises the possibility that CD4 is also present in nerve tissue and that a cell surface receptor for class II major histocompatibility complex antigens could play a role in central nervous system function. This possibility is reinforced by the detection of unique CD4-related transcripts in mouse and human brain tissue. In this study, the structure of the mouse brain CD4 transcript was determined. It is identical to the last two-thirds of the CD4 message and is capable of encoding a 217-residue protein that would consist of a truncated, 154-residue, cell surface region, together with the complete CD4 transmembrane and cytoplasmic regions. It would not include an amino-terminal hydrophobic leader peptide.

DNA-binding site for two skeletal actin promoter factors is important for expression in muscle cells

Two nuclear factors bind to the same site in the chicken skeletal actin promoter. Mutations in the footprint sequence which eliminate detectable binding decrease expression in transfected skeletal muscle cells by a factor of 25 to 50 and do not elevate the low expression in nonmuscle cells. These results show that the factor-binding site contributes to the activation of expression in muscle cells and that it alone does not contribute significantly to repress expression in nonmuscle cells.

Identification of upstream and intragenic regulatory elements that confer cell-type-restricted and differentiation-specific expression on the muscle creatine kinase gene

Terminal differentiation of skeletal myoblasts is accompanied by induction of a series of tissue-specific gene products, which includes the muscle isoenzyme of creatine kinase (MCK). To begin to define the sequences and signals involved in MCK regulation in developing muscle cells, the mouse MCK gene has been isolated. Sequence analysis of 4,147 bases of DNA surrounding the transcription initiation site revealed several interesting structural features, some of which are common to other muscle-specific genes and to cellular and viral enhancers. To test for sequences required for regulated expression, a region upstream of the MCK gene from -4800 to +1 base pairs, relative to the transcription initiation site, was linked to the coding sequences of the bacterial chloramphenicol acetyltransferase (CAT) gene. Introduction of this MCK-CAT fusion gene into C2 muscle cells resulted in high-level expression of CAT activity in differentiated myotubes and no detectable expression in proliferating undifferentiated myoblasts or in nonmyogenic cell lines. Deletion mutagenesis of sequences between -4800 and the transcription start site showed that the region between -1351 and -1050 was sufficient to confer cell type-specific and developmentally regulated expression on the MCK promoter. This upstream regulatory element functioned independently of position, orientation, or distance from the promoter and therefore exhibited the properties of a classical enhancer. This upstream enhancer also was able to confer muscle-specific regulation on the simian virus 40 promoter, although it exhibited a 3- to 5-fold preference for its own promoter. In contrast to the cell type- and differentiation-specific expression of the upstream enhancer, the MCK promoter was able to function in myoblasts and myotubes and in nonmyogenic cell lines when combined with the simian virus 40 enhancer. An additional positive regulatory element was identified within the first intron of the MCK gene. Like the upstream enhancer, this intragenic element functioned independently of position, orientation, and distance with respect to the MCK promoter and was active in differentiated myotubes but not in myoblasts. These results demonstrate that expression of the MCK gene in developing muscle cells is controlled by complex interactions among multiple upstream and intragenic regulatory elements that are functional only in the appropriate cellular context.

Mouse brain CD4 transcripts encode only the COOH-terminal half of the protein

The T-cell surface glycoprotein CD4 is thought to function as a receptor for class II major histocompatibility complex molecules. Human CD4 is also the lymphoid cell receptor for human immunodeficiency virus, the causative agent of acquired immune deficiency syndrome. The observed infection of the central nervous system in acquired immune deficiency syndrome patients raises the possibility that CD4 is also present in nerve tissue and that a cell surface receptor for class II major histocompatibility complex antigens could play a role in central nervous system function. This possibility is reinforced by the detection of unique CD4-related transcripts in mouse and human brain tissue. In this study, the structure of the mouse brain CD4 transcript was determined. It is identical to the last two-thirds of the CD4 message and is capable of encoding a 217-residue protein that would consist of a truncated, 154-residue, cell surface region, together with the complete CD4 transmembrane and cytoplasmic regions. It would not include an amino-terminal hydrophobic leader peptide.

Do exons code for structural or functional units in proteins?

In considering the origin and evolution of proteins, the possibility that proteins evolved from exons coding for specific structure-function modules is attractive for its economy and simplicity but is not systematically supported by the available data. However, the number of correspondences between exons and units of protein structure-function that have so far been identified appears to be greater than expected by chance alone. The available data also show (i) that exons are fairly limited in size but are large enough to specify structure-function modules in proteins; (ii) that the position of introns for homologous domains in the same gene is reasonably stable, but there is also evidence for mechanisms that alter the position or existence of introns; and (iii) that it is possible that the observed relationship of exons to protein structure represents a degenerate state of an ancestral correspondence between exons and structure-function modules in proteins.

DNA-binding site for two skeletal actin promoter factors is important for expression in muscle cells

Two nuclear factors bind to the same site in the chicken skeletal actin promoter. Mutations in the footprint sequence which eliminate detectable binding decrease expression in transfected skeletal muscle cells by a factor of 25 to 50 and do not elevate the low expression in nonmuscle cells. These results show that the factor-binding site contributes to the activation of expression in muscle cells and that it alone does not contribute significantly to repress expression in nonmuscle cells.

Complex regulation of the muscle-specific contractile protein (troponin I) gene

A cloned quail troponin I contractile protein gene, stably transfected into a mouse myogenic cell line, exhibits appropriate developmental activation and quantitative expression during myoblast differentiation. Deletion mutagenesis analyses reveal that the troponin I gene has two distinct cis regulatory elements required for its developmental expression, as measured by mRNA accumulation and nuclear runoff transcription assays. One element in the 5' flanking region is required for maximum quantitative expression, and a second larger regulatory element (1.5 kilobases) within the first intron is responsible for differentiation-specific transcription. The upstream region is highly sensitive to negative repression by interaction with pBR322 sequences. The larger intragenic region retains some activity when moved to the 5' and 3' flanking regions and when inverted but is maximally active in its native intragenic site. The concerted activities of these two regulatory regions produce a 100- to 200-fold transcriptional activation during myoblast differentiation. The conserved 5' exon-intron organization of troponin I and other contractile protein genes suggests a possible mechanism by which intragenic control elements coordinate contractile protein gene regulation during skeletal myogenesis.

Complex regulation of the muscle-specific contractile protein (troponin I) gene

A cloned quail troponin I contractile protein gene, stably transfected into a mouse myogenic cell line, exhibits appropriate developmental activation and quantitative expression during myoblast differentiation. Deletion mutagenesis analyses reveal that the troponin I gene has two distinct cis regulatory elements required for its developmental expression, as measured by mRNA accumulation and nuclear runoff transcription assays. One element in the 5' flanking region is required for maximum quantitative expression, and a second larger regulatory element (1.5 kilobases) within the first intron is responsible for differentiation-specific transcription. The upstream region is highly sensitive to negative repression by interaction with pBR322 sequences. The larger intragenic region retains some activity when moved to the 5' and 3' flanking regions and when inverted but is maximally active in its native intragenic site. The concerted activities of these two regulatory regions produce a 100- to 200-fold transcriptional activation during myoblast differentiation. The conserved 5' exon-intron organization of troponin I and other contractile protein genes suggests a possible mechanism by which intragenic control elements coordinate contractile protein gene regulation during skeletal myogenesis.

Delimitation and characterization of cis-acting DNA sequences required for the regulated expression and transcriptional control of the chicken skeletal alpha-actin gene

We have previously observed that DNA sequences within the 5'-flanking region of the chicken skeletal alpha-actin gene harbor a cis-acting regulatory element that influences cell type and developmental stage-specific expression (J. M. Grichnik, D. J. Bergsma, and R. J. Schwartz, Nucleic Acids Res 14:1683-1701, 1986). In this report we have constructed unidirectional 5'-deletion and region-specific deletion-insertion mutations of the chicken skeletal alpha-actin upstream region and inserted these into the chloramphenicol acetyltransferase expression vector pSV0CAT. These constructions were used to locate DNA sequences that are required for developmental modulation of expression when transfected into differentiating myoblasts. With this assay we have delimited the 5' boundary of a cis-acting regulatory element to ca. 200 base pairs upstream of the mRNA cap site. In addition, we have preliminarily identified DNA sequences that may be important subcomponents within this element. A second major focus of this study was to identify those DNA signals within the regulatory element that control transcription. Toward this end, the expression phenotypes of progressive 5'-deletion and deletion-insertion mutants of the 5'-flanking region of the chicken skeletal alpha-actin gene were assayed in microinjected Xenopus laevis oocytes. These experiments defined a cis-acting transcriptional control region having a 5' border 107 base pairs preceding the alpha-actin RNA cap site. Proximal and distal functionally important regions of DNA were identified within this element. These DNA signals included within their DNA sequences the "CCAAT" and "TATA" box homologies.

Delimitation and characterization of cis-acting DNA sequences required for the regulated expression and transcriptional control of the chicken skeletal alpha-actin gene

We have previously observed that DNA sequences within the 5'-flanking region of the chicken skeletal alpha-actin gene harbor a cis-acting regulatory element that influences cell type and developmental stage-specific expression (J. M. Grichnik, D. J. Bergsma, and R. J. Schwartz, Nucleic Acids Res 14:1683-1701, 1986). In this report we have constructed unidirectional 5'-deletion and region-specific deletion-insertion mutations of the chicken skeletal alpha-actin upstream region and inserted these into the chloramphenicol acetyltransferase expression vector pSV0CAT. These constructions were used to locate DNA sequences that are required for developmental modulation of expression when transfected into differentiating myoblasts. With this assay we have delimited the 5' boundary of a cis-acting regulatory element to ca. 200 base pairs upstream of the mRNA cap site. In addition, we have preliminarily identified DNA sequences that may be important subcomponents within this element. A second major focus of this study was to identify those DNA signals within the regulatory element that control transcription. Toward this end, the expression phenotypes of progressive 5'-deletion and deletion-insertion mutants of the 5'-flanking region of the chicken skeletal alpha-actin gene were assayed in microinjected Xenopus laevis oocytes. These experiments defined a cis-acting transcriptional control region having a 5' border 107 base pairs preceding the alpha-actin RNA cap site. Proximal and distal functionally important regions of DNA were identified within this element. These DNA signals included within their DNA sequences the "CCAAT" and "TATA" box homologies.

Mouse muscle nicotinic acetylcholine receptor gamma subunit: cDNA sequence and gene expression

Clones coding for the mouse nicotinic acetylcholine receptor (AChR) gamma subunit precursor have been selected from a cDNA library derived from a mouse myogenic cell line and sequenced. The deduced protein sequence consists of a signal peptide of 22 amino acid residues and a mature gamma subunit of 497 amino acid residues. There is a high degree of sequence conservation between this mouse sequence and published human and calf AChR gamma subunits and, after allowing for functional amino acid substitutions, also to the more distantly related chicken and Torpedo AChR gamma subunits. The degree of sequence conservation is especially high in the four putative hydrophobic membrane spanning regions, supporting the assignment of these domains. RNA blot hybridization showed that the mRNA level of the gamma subunit increases by 30 fold or more upon differentiation of the two mouse myogenic cell lines, BC3H-1 and C2C12, suggesting that the primary controls for changes in gene expression during differentiation are at the level of transcription. One cDNA clone was found to correspond to a partially processed nuclear transcript containing two as yet unspliced intervening sequences.