Tissue-specific transcription of the cardiac myosin light-chain 2 gene is regulated by an upstream repressor element

Expression of myotilin during chicken development

Several missense mutations in the Z-band protein, myotilin, have been implicated in human muscle diseases such as myofibrillar myopathy, spheroid body myopathy, and distal myopathy. Recently, we have reported the cloning of chicken myotilin cDNA. In this study, we have investigated the expression of myotilin in cross-striated muscles from developing chicken by qRT-PCR and in situ hybridizations. In situ hybridization of embryonic stages shows myotilin gene expression in heart, somites, neural tissue, eyes and otocysts. RT-PCR and qRT-PCR data, together with in situ hybridization results point to a biphasic transcriptional pattern for MYOT gene during early heart development with maximum expression level in the adult. In skeletal muscle, the expression level starts decreasing after embryonic day 20 and declines in the adult skeletal muscles. Western blot assays of myotilin in adult skeletal muscle reveal a decrease in myotilin protein compared with levels in embryonic skeletal muscle. Our results suggest that MYOT gene may undergo transcriptional activation and repression that varies between tissues in developing chicken. We believe this is the first report of the developmental regulation on myotilin expression in non-mammalian species.

Repression of the cardiac myosin light chain-2 gene in skeletal muscle requires site-specific association of antithetic regulator, Nished, and HDACs

The transcriptional activation mechanisms that regulate tissue-specific expression of cardiac muscle genes have been extensively investigated, but little is known of the regulatory events involved in repression of cardiac-specific genes in non-cardiac cells. We have previously reported that Nished, a ubiquitous transcription factor, interacts with a positive sequence element, the Intron Regulatory Element (IRE) as well as a negatively acting element, the Cardiac-Specific Sequence (CSS), in myosin light chain-2 (MLC2v) gene to promote activation and repression of the gene in cardiac and skeletal muscle cells respectively. Here, we show that the negative regulation of cardiac MLC2v gene in skeletal muscle cells is mediated via the interaction of Nished with histone deacetylase (HDAC) co-repressor. Treatment of cells with the HDAC inhibitor, Trichostatin A (TSA), alleviates the repressor activity of Nished in a dose-dependent manner. Co-transfection studies in primary muscle cells in culture and in Nished expressing stable skeletal muscle cell line demonstrate that Nished down-regulates the cardiac MLC2 gene expression when its association is restricted to CSS alone. Chromatin immunoprecipitation data suggest that the CSS-mediated repression of cardiac MLC2v gene in skeletal muscle cells excludes the participation of the positive element IRE despite the presence of an identical Nished binding site. Taken together, it appears that the negative control of MLC2v transcription is based on a dual mode of regulations, one that affords inaccessibility of IRE to Nished and second that promotes the formation of the transcription repression complex at the inhibitory CSS site to silence the cardiac gene in skeletal muscle cell.

Requirement of TCTG(G/C) Direct Repeats and Overlapping GATA Site for Maintaining the Cardiac-Specific Expression of Cardiac troponin T in Developing and Adult Mice

The cardiac-specific -497 bp promoter of rat cardiac troponin T (cTnT) contains two similar modules, D and F, each of which possesses TCTG(G/C) direct repeats and A/T-rich sites. To identify cis-elements critical for cardiac specificity, a -249 bp promoter containing only module F and its site-directed mutations were used to generate transgenic mice. Transgene expression of the -249 bp promoter remained cardiac-specific, despite low and nonuniform expression. The nonuniform expression pattern of the transgene coincided with differential expression of HMGB1, which appeared to be the predominant form of HMGB family proteins in the heart. The HMGB1 binds to the A/T-rich/MEF2-like sites of the cTnT promoter, as determined by chromatin immunoprecipitation assays. Mice carrying the -249 bp promoter with point mutations disrupting the direct repeats expressed transgene at lower levels in the heart and ectopically in the brain. Ectopic expression of transgene was also observed in developing limbs and head. These results suggest an important role for the direct repeat in determining the cardiac specificity. Furthermore, mice carrying a mutant promoter simultaneously disrupting the direct repeats and overlapping GATA site failed to express the transgene in any tissues tested. Therefore, the direct repeat and overlapping GATA site are critical for the expression level and cardiac specificity. The F module controls one level of cardiac specificity. For a uniform and high level of cardiac-specific expression, the upstream element (-497 to -250 bp) is further required, possibly through the D enhancer module and the combination of Nkx2.5 and GATA sites.

Modulation of MLC-2v gene expression by AP-1: complex regulatory role of Jun in cardiac myocytes

Hypertrophic stimulation of cardiac myocytes results in rapid induction of a number of transcription factors, including members of the AP-1 family, which is followed by a programmed alteration in the pattern of gene expression. In the ventricular cardiocytes there is re-expression of the fetal atrial natriuretic factor (ANF) gene and upregulation of its myosin light chain-2 (MLC-2v). The mechanism(s) by which the induction ofAP-1 is coupled to the promoters of these target genes is largely unknown. In this report, we demonstrate that in transient co-transfection assay, c-Jun inhibited while Jun B stimulated the MLC-2v promoter activity. Mutant c-Jun recombinants, in which the activation domains were deleted, still remained inhibitory, but a specific mutation in the leucine zipper, which changes the alignment of Jun with its dimerization partner, caused a reversal of its effect on the target MLC-2v promoter. Based on these findings, we propose that in chicken cardiac myocytes, the regulation of MLC-2v promoter by Jun may occur via its interaction with other proteins, possibly of the leucine zipper family.

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.

Transcription enhancer factor 1 interacts with a basic helix-loop-helix zipper protein, Max, for positive regulation of cardiac alpha-myosin heavy-chain gene expression

The M-CAT binding factor transcription enhancer factor 1 (TEF-1) has been implicated in the regulation of several cardiac and skeletal muscle genes. Previously, we identified an E-box-M-CAT hybrid (EM) motif that is responsible for the basal and cyclic AMP-inducible expression of the rat cardiac alpha-myosin heavy chain (alpha-MHC) gene in cardiac myocytes. In this study, we report that two factors, TEF-1 and a basic helix-loop-helix leucine zipper protein, Max, bind to the alpha-MHC EM motif. We also found that Max was a part of the cardiac troponin T M-CAT-TEF-1 complex even when the DNA template did not contain an apparent E-box binding site. In the protein-protein interaction assay, a stable association of Max with TEF-1 was observed when glutathione S-transferase (GST)-TEF-1 or GST-Max was used to pull down in vitro-translated Max or TEF-1, respectively. In addition, Max was coimmunoprecipitated with TEF-1, thus documenting an in vivo TEF-1-Max interaction. In the transient transcription assay, overexpression of either Max or TEF-1 resulted a mild activation of the alpha-MHC-chloramphenicol acetyltransferase (CAT) reporter gene at lower concentrations and repression of this gene at higher concentrations. However, when Max and TEF-1 expression plasmids were transfected together, the repression mediated by a single expression plasmid was alleviated and a three- to fourfold transactivation of the alpha-MHC-CAT reporter gene was observed. This effect was abolished once the EM motif in the promoter-reporter construct was mutated, thus suggesting that the synergistic transactivation function of the TEF-1-Max heterotypic complex is mediated through binding of the complex to the EM motif. These results demonstrate a novel association between Max and TEF-1 and indicate a positive cooperation between these two factors in alpha-MHC gene regulation.

Differential expression of the myocyte enhancer factor 2 family of transcription factors in development: the cardiac factor BBF-1 is an early marker for cardiogenesis

In the present study, we have used single chicken blastoderms of defined early developmental stages, beginning with the prestreak stage, stage 1 (V. Hamburger and H. L. Hamilton, J. Morphol. 88:49-92, 1951), to analyze the onset of cardiac myogenesis by monitoring the appearance of selected cardiac muscle tissue-specific gene transcripts and the functional expression of the myocyte enhancer factor 2 (MEF-2) proteins. Using gene-specific oligonucleotide primers in reverse transcriptase PCR assay, we have demonstrated that the cardiac myosin light-chain 2 (MLC2) and alpha-actin gene transcripts appear as early as stage 5, i.e., immediately after the cardiogenic fate assignment at stage 4. Consistent with this observation is the developmental expression pattern of DNA-binding activity of BBF-1, a cardiac muscle-specific member of the MEF-2 protein family, which also begins at stage 5 prior to MEF-2. Differential expression of DNA-binding complexes is also observed with another AT-rich DNA sequence (CArG box) as probe, but the binding pattern with the ubiquitous TATA-binding proteins remains unchanged during the same developmental period. Thus, the cardiogenic commitment and differentiation of the precardiac mesoderm, as exemplified by the appearance of cardiac MEF-2, MLC2, and alpha-actin gene products, occur earlier than previously thought and appear to be closely linked. The onset of skeletal myogenic program follows that of the cardiogenic program with the appearance of skeletal MLC2 at stage 8. We also observed that mRNA for the MEF-2 family of proteins appears as early as stage 2 and that for CMD-1, the chicken counterpart of MyoD, appears at stage 5. The temporal separation of activation of cardiac and skeletal MLC2 genes, which appears immediately after the respective fate assignments, and those of cardiac MEF-2 and CMD-1, which occur before, are consistent with the established appearance of the myogenic programs and with the acquisition pattern of the two tissue-specific morphological characteristics in the early embryo. The preferential appearance of BBF-1 activity in precardiac moesderm, relative to that of MEF-2, indicates that these two protein factors are distinct members of the MEF-2 family and provides a compelling argument in support of the potential role of BBF-1 as a regulator of the cardiogenic cell lineage determination, while cardiac MEF-2 might be involved in maintenance of the cardiac differentiative state.

Differential expression of the myocyte enhancer factor 2 family of transcription factors in development: the cardiac factor BBF-1 is an early marker for cardiogenesis

In the present study, we have used single chicken blastoderms of defined early developmental stages, beginning with the prestreak stage, stage 1 (V. Hamburger and H. L. Hamilton, J. Morphol. 88:49-92, 1951), to analyze the onset of cardiac myogenesis by monitoring the appearance of selected cardiac muscle tissue-specific gene transcripts and the functional expression of the myocyte enhancer factor 2 (MEF-2) proteins. Using gene-specific oligonucleotide primers in reverse transcriptase PCR assay, we have demonstrated that the cardiac myosin light-chain 2 (MLC2) and alpha-actin gene transcripts appear as early as stage 5, i.e., immediately after the cardiogenic fate assignment at stage 4. Consistent with this observation is the developmental expression pattern of DNA-binding activity of BBF-1, a cardiac muscle-specific member of the MEF-2 protein family, which also begins at stage 5 prior to MEF-2. Differential expression of DNA-binding complexes is also observed with another AT-rich DNA sequence (CArG box) as probe, but the binding pattern with the ubiquitous TATA-binding proteins remains unchanged during the same developmental period. Thus, the cardiogenic commitment and differentiation of the precardiac mesoderm, as exemplified by the appearance of cardiac MEF-2, MLC2, and alpha-actin gene products, occur earlier than previously thought and appear to be closely linked. The onset of skeletal myogenic program follows that of the cardiogenic program with the appearance of skeletal MLC2 at stage 8. We also observed that mRNA for the MEF-2 family of proteins appears as early as stage 2 and that for CMD-1, the chicken counterpart of MyoD, appears at stage 5. The temporal separation of activation of cardiac and skeletal MLC2 genes, which appears immediately after the respective fate assignments, and those of cardiac MEF-2 and CMD-1, which occur before, are consistent with the established appearance of the myogenic programs and with the acquisition pattern of the two tissue-specific morphological characteristics in the early embryo. The preferential appearance of BBF-1 activity in precardiac moesderm, relative to that of MEF-2, indicates that these two protein factors are distinct members of the MEF-2 family and provides a compelling argument in support of the potential role of BBF-1 as a regulator of the cardiogenic cell lineage determination, while cardiac MEF-2 might be involved in maintenance of the cardiac differentiative state.

A novel myogenic regulatory circuit controls slow/cardiac troponin C gene transcription in skeletal muscle

The slow/cardiac troponin C (cTnC) gene is expressed in three distinct striated muscle lineages: cardiac myocytes, embryonic fast skeletal myotubes, and adult slow skeletal myocytes. We have reported previously that cTnC gene expression in cardiac muscle is regulated by a cardiac-specific promoter/enhancer located in the 5' flanking region of the gene (bp -124 to +1). In this report, we demonstrate that the cTnC gene contains a second distinct and independent transcriptional enhancer which is located in the first intron. This second enhancer is skeletal myotube specific and is developmentally up-regulated during the differentiation of myoblasts to myotubes. This enhancer contains three functionally important nuclear protein binding sites: a CACCC box, a MEF-2 binding site, and a previously undescribed nuclear protein binding site, designated MEF-3, which is also present in a large number of skeletal muscle-specific transcriptional enhancers. Unlike most skeletal muscle-specific transcriptional regulatory elements, the cTnC enhancer does not contain a consensus binding site (CANNTG) for the basic helix-loop-helix (bHLH) family of transcription factors and does not directly bind MyoD-E12 protein complexes. Despite these findings, the cTnC enhancer can be transactivated by overexpression of the myogenic bHLH proteins, MyoD and myogenin, in C3H10T1/2 (10T1/2) cells. Electrophoretic mobility shift assays demonstrated changes in the patterns of MEF-2, CACCC, and MEF-3 DNA binding activities following the conversion of 10T1/2 cells into myoblasts and myotubes by stable transfection with a MyoD expression vector. In particular, MEF-2 binding activity was up-regulated in 10T1/2 cells stably transfected with a MyoD expression vector only after these cells fused and differentiated into skeletal myotubes. Taken together, these results demonstrated that distinct lineage-specific transcriptional regulatory elements control the expression of a single myofibrillar protein gene in fast skeletal and cardiac muscle. In addition, they show that bHLH transcription factors can indirectly transactivate the expression of some muscle-specific genes.

A novel myogenic regulatory circuit controls slow/cardiac troponin C gene transcription in skeletal muscle

The slow/cardiac troponin C (cTnC) gene is expressed in three distinct striated muscle lineages: cardiac myocytes, embryonic fast skeletal myotubes, and adult slow skeletal myocytes. We have reported previously that cTnC gene expression in cardiac muscle is regulated by a cardiac-specific promoter/enhancer located in the 5' flanking region of the gene (bp -124 to +1). In this report, we demonstrate that the cTnC gene contains a second distinct and independent transcriptional enhancer which is located in the first intron. This second enhancer is skeletal myotube specific and is developmentally up-regulated during the differentiation of myoblasts to myotubes. This enhancer contains three functionally important nuclear protein binding sites: a CACCC box, a MEF-2 binding site, and a previously undescribed nuclear protein binding site, designated MEF-3, which is also present in a large number of skeletal muscle-specific transcriptional enhancers. Unlike most skeletal muscle-specific transcriptional regulatory elements, the cTnC enhancer does not contain a consensus binding site (CANNTG) for the basic helix-loop-helix (bHLH) family of transcription factors and does not directly bind MyoD-E12 protein complexes. Despite these findings, the cTnC enhancer can be transactivated by overexpression of the myogenic bHLH proteins, MyoD and myogenin, in C3H10T1/2 (10T1/2) cells. Electrophoretic mobility shift assays demonstrated changes in the patterns of MEF-2, CACCC, and MEF-3 DNA binding activities following the conversion of 10T1/2 cells into myoblasts and myotubes by stable transfection with a MyoD expression vector. In particular, MEF-2 binding activity was up-regulated in 10T1/2 cells stably transfected with a MyoD expression vector only after these cells fused and differentiated into skeletal myotubes. Taken together, these results demonstrated that distinct lineage-specific transcriptional regulatory elements control the expression of a single myofibrillar protein gene in fast skeletal and cardiac muscle. In addition, they show that bHLH transcription factors can indirectly transactivate the expression of some muscle-specific genes.

A new serum-responsive, cardiac tissue-specific transcription factor that recognizes the MEF-2 site in the myosin light chain-2 promoter

We have identified a serum-responsive, cardiac tissue-specific transcription factor, BBF-1, that recognizes an AT-rich sequence (element B), identical to the myocyte enhancer factor (MEF-2) target site, in the cardiac myosin light chain-2 (MLC-2) promoter. Deletion of the element B sequence alone from the cardiac MLC-2 promoter causes, as does that of the MEF-2 site from other promoters and the enhancer of skeletal muscle genes, a marked reduction of transcription. BBF-1 is distinguishable from cardiac MEF-2 on the basis of immunoprecipitation with an antibody which recognizes MEF-2 but not BBF-1. Unlike MEF-2, BBF-1 is present exclusively in nuclear extracts from cardiac muscle cells cultured in a medium containing a high concentration of serum. Removal of serum from culture medium abolishes BBF-1 activity selectively with a concomitant loss of the positive regulatory effect of element B on MLC-2 gene transcription, indicating that there is a correlation between the BBF-1 binding activity and the tissue-specific role of the element B (MEF-2 site) sequence. The loss of element B-mediated activation of transcription is reversed following the refeeding of cells with serum-containing medium. These data demonstrate that cardiac muscle cells contain two distinct protein factors, MEF-2 and BBF-1, which bind to the same target site but that, unlike MEF-2, BBF-1 is serum inducible and cardiac tissue specific. BBF-1 thus appears to be a crucial member of the MEF-2 family of proteins which will serve as an important tool in understanding the regulatory mechanism(s) underlying cardiogenic differentiation.

Identification of a functional silencer element involved in neuron-specific expression of the synapsin I gene

We have identified a functional silencer element (positions -231 to -211) in the human synapsin I gene that selectively represses its transcription in nonneuronal cells. Transfection experiments using synapsin I-luciferase constructs show that site-specific mutations or deletion of this silencer sequence results in expression of the reporter gene in nonneuronal cells. Moreover, the silencer element is capable of conferring repression on a heterologous promoter in nonneuronal cells. Gel-shift assays reveal the presence of a sequence-specific synapsin I silencer-binding protein in nonneuronal cell extracts but not in neuronal cell extracts. Mutagenesis studies of the silencer sequence demonstrate that formation of the specific silencer-protein complex in vitro correlates well with repression of transcription in vivo. These data indicate that the interaction between synapsin I silencer and its binding protein is involved in tissue-specific expression of the synapsin I gene. In addition, our results suggest the existence of at least one additional cis-acting element within the promoter-proximal region (positions -233 to +20) that also contributes to the neuron-specific expression of the synapsin I gene.

A new serum-responsive, cardiac tissue-specific transcription factor that recognizes the MEF-2 site in the myosin light chain-2 promoter

We have identified a serum-responsive, cardiac tissue-specific transcription factor, BBF-1, that recognizes an AT-rich sequence (element B), identical to the myocyte enhancer factor (MEF-2) target site, in the cardiac myosin light chain-2 (MLC-2) promoter. Deletion of the element B sequence alone from the cardiac MLC-2 promoter causes, as does that of the MEF-2 site from other promoters and the enhancer of skeletal muscle genes, a marked reduction of transcription. BBF-1 is distinguishable from cardiac MEF-2 on the basis of immunoprecipitation with an antibody which recognizes MEF-2 but not BBF-1. Unlike MEF-2, BBF-1 is present exclusively in nuclear extracts from cardiac muscle cells cultured in a medium containing a high concentration of serum. Removal of serum from culture medium abolishes BBF-1 activity selectively with a concomitant loss of the positive regulatory effect of element B on MLC-2 gene transcription, indicating that there is a correlation between the BBF-1 binding activity and the tissue-specific role of the element B (MEF-2 site) sequence. The loss of element B-mediated activation of transcription is reversed following the refeeding of cells with serum-containing medium. These data demonstrate that cardiac muscle cells contain two distinct protein factors, MEF-2 and BBF-1, which bind to the same target site but that, unlike MEF-2, BBF-1 is serum inducible and cardiac tissue specific. BBF-1 thus appears to be a crucial member of the MEF-2 family of proteins which will serve as an important tool in understanding the regulatory mechanism(s) underlying cardiogenic differentiation.

Identification of a negative element in the human vimentin promoter: modulation by the human T-cell leukemia virus type I Tax protein

The vimentin gene is a member of the intermediate filament multigene family and encodes a protein expressed, in vivo, in all mesenchymal derivatives and, in vitro, in cell types of various origin. We have previously demonstrated that the expression of this growth-regulated gene could be trans activated by the 40-kDa Tax protein of HTLV-I (human T-cell leukemia virus type I) and that responsiveness to this viral protein was mediated by the presence of an NF-kappa B binding site located between -241 and -210 bp upstream of the mRNA cap site (A. Lilienbaum, M. Duc Dodon, C. Alexandre, L. Gazzolo, and D. Paulin, J. Virol. 64:256-263, 1990). These previous assays, performed with deletion mutants of the vimentin promoter linked to the chloramphenicol acetyltransferase gene, also revealed the presence of an upstream negative region between -529 and -241 bp. Interestingly, the inhibitory activity exerted by this negative region was overcome after cotransfection of a Tax-expressing plasmid. In this study, we further characterize the vimentin negative element and define the effect of the Tax protein on the inhibitory activity of this element. We first demonstrate that a 187-bp domain (-424 to -237 bp) behaves as a negative region when placed upstream either of the NF-kappa B binding site of vimentin or of a heterologous enhancer such as that present in the desmin gene promoter. The negative effect can be further assigned to a 32-bp element which is indeed shown to repress the basal or induced activity of the NF-kappa B binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of a negative element in the human vimentin promoter: modulation by the human T-cell leukemia virus type I Tax protein

The vimentin gene is a member of the intermediate filament multigene family and encodes a protein expressed, in vivo, in all mesenchymal derivatives and, in vitro, in cell types of various origin. We have previously demonstrated that the expression of this growth-regulated gene could be trans activated by the 40-kDa Tax protein of HTLV-I (human T-cell leukemia virus type I) and that responsiveness to this viral protein was mediated by the presence of an NF-kappa B binding site located between -241 and -210 bp upstream of the mRNA cap site (A. Lilienbaum, M. Duc Dodon, C. Alexandre, L. Gazzolo, and D. Paulin, J. Virol. 64:256-263, 1990). These previous assays, performed with deletion mutants of the vimentin promoter linked to the chloramphenicol acetyltransferase gene, also revealed the presence of an upstream negative region between -529 and -241 bp. Interestingly, the inhibitory activity exerted by this negative region was overcome after cotransfection of a Tax-expressing plasmid. In this study, we further characterize the vimentin negative element and define the effect of the Tax protein on the inhibitory activity of this element. We first demonstrate that a 187-bp domain (-424 to -237 bp) behaves as a negative region when placed upstream either of the NF-kappa B binding site of vimentin or of a heterologous enhancer such as that present in the desmin gene promoter. The negative effect can be further assigned to a 32-bp element which is indeed shown to repress the basal or induced activity of the NF-kappa B binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of cis-acting elements regulating transcription from the promoter of a constitutively active rice actin gene

The promoter of the constitutively expressed rice (Oryza sativa) actin 1 gene (Act1) is highly active in transformed rice plants (W. Zhang, D. McElroy, and R. Wu, Plant Cell 3:1150-1160, 1991). A region 834 bp upstream of the Act1 transcription initiation site contains all the regulatory elements necessary for maximal gene expression in transformed rice protoplasts (D. McElroy, W. Zhang, J. Cao, and R. Wu, Plant Cell 2:163-171, 1990). We have constructed a series of Act1 promoter deletions fused to a bacterial beta-glucuronidase reporter sequence (Gus). Transient expression assays in transformed rice protoplasts, as well as transformed maize cells and tissues, identified two distinct cis-acting regulatory elements in the Act1 promoter. A 38-bp poly(dA-dT) region was found to be a positive regulator of Act1 promoter activity. Deletion of the poly(dA-dT) element lowered Gus expression by at least threefold compared with expression produced by the full-length Act1 promoter. By gel retardation and footprinting, we identified a ubiquitous rice protein which specifically recognizes this poly(dA-dT) element in the constitutively active Act1 promoter. A CCCAA pentamer repeat-containing region was found to be a negative regulator of the Act1 promoter in transformed rice protoplasts. Transient expression assays in different maize cells and tissues with use of the Act1 deletion constructs suggested that the CCCAA pentamer repeat region functions in a complex tissue-specific manner. A CCCAA-binding protein was detected only in root extracts.

Characterization of cis-acting elements regulating transcription from the promoter of a constitutively active rice actin gene

The promoter of the constitutively expressed rice (Oryza sativa) actin 1 gene (Act1) is highly active in transformed rice plants (W. Zhang, D. McElroy, and R. Wu, Plant Cell 3:1150-1160, 1991). A region 834 bp upstream of the Act1 transcription initiation site contains all the regulatory elements necessary for maximal gene expression in transformed rice protoplasts (D. McElroy, W. Zhang, J. Cao, and R. Wu, Plant Cell 2:163-171, 1990). We have constructed a series of Act1 promoter deletions fused to a bacterial beta-glucuronidase reporter sequence (Gus). Transient expression assays in transformed rice protoplasts, as well as transformed maize cells and tissues, identified two distinct cis-acting regulatory elements in the Act1 promoter. A 38-bp poly(dA-dT) region was found to be a positive regulator of Act1 promoter activity. Deletion of the poly(dA-dT) element lowered Gus expression by at least threefold compared with expression produced by the full-length Act1 promoter. By gel retardation and footprinting, we identified a ubiquitous rice protein which specifically recognizes this poly(dA-dT) element in the constitutively active Act1 promoter. A CCCAA pentamer repeat-containing region was found to be a negative regulator of the Act1 promoter in transformed rice protoplasts. Transient expression assays in different maize cells and tissues with use of the Act1 deletion constructs suggested that the CCCAA pentamer repeat region functions in a complex tissue-specific manner. A CCCAA-binding protein was detected only in root extracts.

A single transcription factor binds to two divergent sequence elements with a common function in cardiac myosin light chain-2 promoter

The cardiac myosin light chain-2 (MLC-2) gene promoter contains several positive and negative cis-acting sequences that are involved in the regulation of its expression. We describe here the properties of two activator sequences, elements A and P, and their DNA-binding factors (ABFs). Element A (CCAAAAGTGG), located at -61, has homology with the evolutionarily conserved sequence CC(A/T)6GG, present in the genes of many contractile proteins. Element P (TAACCTTGAAAGC), located 114 bp upstream of element A, is conserved in both chicken and rat cardiac MLC-2 gene promoters. Deletion mutagenesis demonstrated that these two elements are involved in the positive regulation of MLC-2 gene transcription. At least two sequence-specific element A-binding proteins, ABF-1 and ABF-2, were identified by gel shift analysis of the fractionated cardiac nuclear proteins. ABF-1 binds to element A with strict dependence on the internal element A sequence AAAAGT. In contrast, ABF-2 exhibits a relaxed sequence requirement, as it recognizes the consensus CArG and CCAAT box sequences as well. ABF-2 also recognizes the distal element P despite the fact that the sequences of elements A and P are divergent. DNase I footprinting, methylation interference, and gel shift analyses demonstrated unequivocally that the element A-DNA affinity-purified protein ABF-2 binds to element P with sequence specificity. Since both elements A and P play a positive regulatory role in MLC-2 gene transcription and bind to a single protein (ABF-2), it would appear that ABF-2 is a key transcription factor with the ability to recognize divergent sequence elements involved in a common regulatory pathway during myogenesis.

A single transcription factor binds to two divergent sequence elements with a common function in cardiac myosin light chain-2 promoter

The cardiac myosin light chain-2 (MLC-2) gene promoter contains several positive and negative cis-acting sequences that are involved in the regulation of its expression. We describe here the properties of two activator sequences, elements A and P, and their DNA-binding factors (ABFs). Element A (CCAAAAGTGG), located at -61, has homology with the evolutionarily conserved sequence CC(A/T)6GG, present in the genes of many contractile proteins. Element P (TAACCTTGAAAGC), located 114 bp upstream of element A, is conserved in both chicken and rat cardiac MLC-2 gene promoters. Deletion mutagenesis demonstrated that these two elements are involved in the positive regulation of MLC-2 gene transcription. At least two sequence-specific element A-binding proteins, ABF-1 and ABF-2, were identified by gel shift analysis of the fractionated cardiac nuclear proteins. ABF-1 binds to element A with strict dependence on the internal element A sequence AAAAGT. In contrast, ABF-2 exhibits a relaxed sequence requirement, as it recognizes the consensus CArG and CCAAT box sequences as well. ABF-2 also recognizes the distal element P despite the fact that the sequences of elements A and P are divergent. DNase I footprinting, methylation interference, and gel shift analyses demonstrated unequivocally that the element A-DNA affinity-purified protein ABF-2 binds to element P with sequence specificity. Since both elements A and P play a positive regulatory role in MLC-2 gene transcription and bind to a single protein (ABF-2), it would appear that ABF-2 is a key transcription factor with the ability to recognize divergent sequence elements involved in a common regulatory pathway during myogenesis.

A position-dependent silencer plays a major role in repressing alpha-fetoprotein expression in human hepatoma

A large percentage of human hepatomas produce alpha-fetoprotein (AFP), but the levels of AFP expression vary greatly among hepatomas. To understand the molecular basis for this variation, we analyzed transcriptional regulatory activities associated with the 5'-flanking region of the AFP gene in two human hepatoma cell lines, HuH-7 and huH-1/cl-2, which produce a high and a low level of AFP, respectively. We found that the low level of AFP production in huH-1/cl-2 is due to the action of at least two silencer regions located between the enhancer and the promoter of the AFP gene. In contrast, no silencer activity is expressed in HuH-7. We identified 5'-CTTCATAACTAATACTT-3' to be a core sequence responsible for the negative regulatory activity. This sequence is repeated four times in a strong, distal silencer region, Sd, whereas one copy is present in a weak, proximal silencer region, Sp. The silencer reduces transcriptional initiation by blocking enhancer activation of the AFP promoter in a position-dependent manner. The silencer functions in the presence of positive transcription factors and may play a key role in developmental repression as well as variable expression of the AFP gene in hepatomas.

A position-dependent silencer plays a major role in repressing alpha-fetoprotein expression in human hepatoma

A large percentage of human hepatomas produce alpha-fetoprotein (AFP), but the levels of AFP expression vary greatly among hepatomas. To understand the molecular basis for this variation, we analyzed transcriptional regulatory activities associated with the 5'-flanking region of the AFP gene in two human hepatoma cell lines, HuH-7 and huH-1/cl-2, which produce a high and a low level of AFP, respectively. We found that the low level of AFP production in huH-1/cl-2 is due to the action of at least two silencer regions located between the enhancer and the promoter of the AFP gene. In contrast, no silencer activity is expressed in HuH-7. We identified 5'-CTTCATAACTAATACTT-3' to be a core sequence responsible for the negative regulatory activity. This sequence is repeated four times in a strong, distal silencer region, Sd, whereas one copy is present in a weak, proximal silencer region, Sp. The silencer reduces transcriptional initiation by blocking enhancer activation of the AFP promoter in a position-dependent manner. The silencer functions in the presence of positive transcription factors and may play a key role in developmental repression as well as variable expression of the AFP gene in hepatomas.

Myogenin induces the myocyte-specific enhancer binding factor MEF-2 independently of other muscle-specific gene products

The myocyte-specific enhancer-binding factor MEF-2 is a nuclear factor that interacts with a conserved element in the muscle creatine kinase and myosin light-chain 1/3 enhancers (L. A. Gossett, D. J. Kelvin, E. A. Sternberg, and E. N. Olson, Mol. Cell. Biol. 9:5022-5033, 1989). We show in this study that MEF-2 is regulated by the myogenic regulatory factor myogenin and that mitogenic signals block this regulatory interaction. Induction of MEF-2 by myogenin occurs in transfected 10T1/2 cells that have been converted to myoblasts by myogenin, as well as in CV-1 kidney cells that do not activate the myogenic program in response to myogenin. Through mutagenesis of the MEF-2 site, we further defined the binding site requirements for MEF-2 and identified potential MEF-2 sites within numerous muscle-specific regulatory regions. The MEF-2 site was also found to bind a ubiquitous nuclear factor whose binding specificity was similar to but distinct from that of MEF-2. Our results reveal that MEF-2 is controlled, either directly or indirectly, by a myogenin-dependent regulatory pathway and suggest that growth factor signals suppress MEF-2 expression through repression of myogenin expression or activity. The ability of myogenin to induce MEF-2 activity in CV-1 cells, which do not activate downstream genes associated with terminal differentiation, also demonstrates that myogenin retains limited function within cell types that are nonpermissive for myogenesis and suggests that MEF-2 is regulated independently of other muscle-specific genes.

Myogenin induces the myocyte-specific enhancer binding factor MEF-2 independently of other muscle-specific gene products

The myocyte-specific enhancer-binding factor MEF-2 is a nuclear factor that interacts with a conserved element in the muscle creatine kinase and myosin light-chain 1/3 enhancers (L. A. Gossett, D. J. Kelvin, E. A. Sternberg, and E. N. Olson, Mol. Cell. Biol. 9:5022-5033, 1989). We show in this study that MEF-2 is regulated by the myogenic regulatory factor myogenin and that mitogenic signals block this regulatory interaction. Induction of MEF-2 by myogenin occurs in transfected 10T1/2 cells that have been converted to myoblasts by myogenin, as well as in CV-1 kidney cells that do not activate the myogenic program in response to myogenin. Through mutagenesis of the MEF-2 site, we further defined the binding site requirements for MEF-2 and identified potential MEF-2 sites within numerous muscle-specific regulatory regions. The MEF-2 site was also found to bind a ubiquitous nuclear factor whose binding specificity was similar to but distinct from that of MEF-2. Our results reveal that MEF-2 is controlled, either directly or indirectly, by a myogenin-dependent regulatory pathway and suggest that growth factor signals suppress MEF-2 expression through repression of myogenin expression or activity. The ability of myogenin to induce MEF-2 activity in CV-1 cells, which do not activate downstream genes associated with terminal differentiation, also demonstrates that myogenin retains limited function within cell types that are nonpermissive for myogenesis and suggests that MEF-2 is regulated independently of other muscle-specific genes.