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The EMBO Journal vol. 1 1 no. 5 pp. 1 843 - 1855, 1992
Co-operativity of functional domains in the muscle-specific transcription factor Myf-5
Barbara Winter, Thomas Braun and Hans-Henning Arnold Department of Toxicology, University of Hamburg, Medical School, Grindelallee 117, 2000 Hamburg 13, FRG Communicated by P.Gruss
Myf-5 is a member of a family of muscle-specific transcription factors that activate myogenesis in 1OT1/2 fibroblasts. Here we report the analysis of Myf-5 structural domains that are responsible for its biological activity. Site-directed mutagenesis revealed that two clusters of basic amino acids within a conserved basic region and two amphipathic helices within the adjacent HLH domain are essential for sequence-specific DNA binding and hetero-oligomerization, respectively. Transcriptional activation by Myf-5 requires two additional domains located in the amino- and carboxyltermini. The two domains apparantly co-operate since deletion of either one results in inactivation. Chimeric proteins between the DNA binding domain of the yeast transcription factor GAL4 and the separate Myf-5 transactivator domains exhibit activity that is enhanced when both regions are combined. Dimerization of Myf-5 with the ubiquitously expressed bHLH protein E12 not only increases the affinity for DNA but also stimulates transactivation independently of DNA binding. The Myf-5 transactivator domains are dependent for activity on a specific amino acid sequence motif within the basic region when Myf-5 activity is mediated through the E-box DNA recognition sequence but not when DNA binding occurs through the GAL4 DNA binding domain. This demonstrates that muscle-specific transactivation by Myf-5 requires the collaboration of two activation domains and the DNA binding region in addition to sequence-specific DNA binding. Transcriptional activation and interaction with DNA are executed by separable domains; however, transactivation is influenced by the basic region in a manner distinguishable from DNA
binding. Key words: bHLH domain/in vitro mutagenesis/muscle differentiation/Myf-5/transcription factor
Introduction The identification of a family of muscle-specific regulatory genes has initiated investigations of the molecular events involved in establishing a cellular program that ultimately leads to the differentiated muscle phenotype. Four related protein factors, MyoDl (Davies et al., 1987), myogenin (Braun et al., 1989a; Edmondson and Olson, 1989; Wright et al., 1989), Myf-5 (Braun et al., 1989b) and MRF4/herculin/Myf-6 (Rhodes and Konieczyn, 1989; Miner and Wold, 1990; Braun et al., 1990a), have been isolated by © Oxford University Press
molecular cloning of their corresponding cDNAs. Constitutive expression of any one of these proteins was shown to convert mouse C3H lOT1/2 fibroblasts and a variety of other non-mesodermal cell lines (Weintraub et al., 1989) to the muscle phenotype (Davis et al., 1987; for review see Olson, 1990; Weintraub et al., 1991a). The myogenic control factors are nuclear phosphoproteins that are expressed only in skeletal muscle cells. They share a highly homologous structural motif encoded by a contiguous stretch of 70 amino acids. This sequence encompasses a cluster of basic amino acids and an adjacent sequence thought to form two amphipathic a-helices connected by a short intervening loop structure. The basic helix-loop-helix domain (bHLH) is a hallmark of a large family of DNA binding proteins including development control genes of Drosophila (Rushlow et al., 1989; Villares and Cabrera, 1987; Caudy et al., 1988; Cronmiller et al., 1988; Thisse et al., 1988; reviewed in Emerson, 1990), the products of the myc oncogene family (Bernard et al., 1983) and numerous ubiquitously expressed transcription factors (Church et al., 1985; Murre et al., 1989a,b; Henthorn et al., 1990). The basic region and the HLH motif mediate DNA binding and dimerization, respectively (Murre et al., 1989b; Voronova and Baltimore, 1990; Davis et al., 1990). Activation of gene transcription by members of the HLH protein family depends on their binding to a DNA consensus motif, referred to as the E-box (CANNTG). This DNA binding site is present in many muscle-specific but also in non-muscle promoters and enhancers (Buskin and Hauschka, 1989) and has been shown to be involved in cell type-specific expression of the genes encoding creatine kinase (MCK) (Lassar et al., 1989), myosin light chain (MCL1/3) (Rosenthal et al., 1990; Braun et al., 1990a), troponin I (Yutzey et al., 1990), actin (Sartorelli et al., 1990) and the a-subunit of the acetylcholine receptor (Piette et al., 1990). Cell type-specific HLH proteins, such as the myogenic factors and the Drosophila proteins of the achaete-scute (AC-S) complex bind weakly to the E-box motif as homomers in vitro, but their affinity to this DNA site is greatly enhanced by the formation of hetero-oligomers with the more widely expressed bHLH proteins E12/E47 encoded by the E2A gene or the homologous protein encoded by the Drosophila gene daugtherless (Murre et al., 1989a,b; Brennan and Olson, 1990; Braun et al., 1990b; Lin et al., 1991: for reviews see Campos-Ortega and Knust, 1989; Simpson, 1990). Recently, it has also been shown that E2A gene products associate with MyoDl and myogenin in vivo and that this oligomerization appears to be essential for executing the myogenic program (Lassar et al., 1991). Reporter genes containing the appropriate DNA binding sites can be transactivated in non-muscle cells by transient expression of the individual muscle regulatory bHLH proteins (for reviews see Olson, 1990; Emerson, 1990; Weintraub et al., 1991a). Although the natural target genes for the four muscle control factors have not been identified -
1843
B.Winter, T.Braun and H.-H.Arnold
(Braun et al., 1990b), suggesting that this protein may require more than the bHLH region for its biological activity. For MyoD it was also demonstrated that the aminoterminus can confer transcriptional activation onto the heterologous DNA binding domain of the yeast transcription factor GALA (Weintraub et al., 199 1b). Paradoxically, different HLH proteins can bind to the same E-box consensus sequence in vitro but do not necessarily activate the corresponding genes. For instance, the E2A gene products E12 and E47 can bind to the E-boxes present in muscle genes but they do not activate transcription of these genes nor do they induce myogenesis in lOT 1/2 cells (Murre et al., 1989b; Davis et al., 1990). This indicates that appropriate DNA binding may not be sufficient for specific gene activation. Experiments with MyoD 1 (Davis et al., 1990) and myogenin (Brennan et al., 1991) have provided evidence that muscle-specific gene activation requires the appropriate basic region not only for DNA binding but also for transcriptional activation. MyoD and myogenin in which the basic regions have been exchanged for those of E12 or the neurogenic HLH protein achaete scute T4 exhibit nearly
in detail, the available evidence suggests that myogenesis is fundamentally associated with the activity of MyoD 1 and its relatives as transcription factors. However, the mechanisms by which these proteins activate transcription and how they exert muscle gene specificity has not been elucidated. Deletion mutagenesis of MyoD revealed that the conserved bHLH region is necessary and sufficient for myogenesis in stably transfected lOT1/2 mouse embryo fibroblasts (Tapscott et al., 1988). This notion suggests that the bHLH region of MyoD contains a transactivator domain or alternatively recruits its activity from the dimerization partners E12 or E47 which have both been shown to contain transcriptional activation domains (Braun et al., 1990b; Henthorn et al., 1990). However, the interpretation of this result has been complicated by the fact that myogenic conversion of lOT1/2 cells is generally associated with the activation of the endogenous bHLH genes by auto- and crossregulatory events (Thayer et al., 1989; Braun et al., 1989b). We have recently reported that a transcriptional activator domain in Myf-5 is located outside of the bHLH domain
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the same DNA binding properties as wild-type proteins in vitro but fail to activate transcription and myogenesis. The mechanism by which the basic region contributes to musclespecific transcription is unknown. The myogenic control protein Myf-5 has attracted our special interest since it is the first protein within the MyoD 1 family that is transiently expressed in dermomyotomal cells during early mouse embryogenesis, suggesting that it may serve aspects in myogenesis that are distinct from MyoD1 and the other factors (Ott et al., 1991). In order to get more insights into the mechanism by which Myf-5 exerts its biological functions, we have employed in vitro mutagenesis to assign biochemical properties to distinct structural features of this protein. Here we report the examination of Myf-5 mutant proteins which were affected in their ability to bind to DNA, to heterodimerize or to activate transcription. We demonstrate that two transcription activation domains of Myf-5 co-operate with each other and with the basic region to induce muscle-specific transcription.
Results The structural domains of Myf-5 that mediate its functions as a muscle-specific transcription factor have not yet been defined. As a general strategy to identify the structures that are required for heterodimerization, DNA binding and myogenic activation, we first introduced point mutations in the putative HLH and basic domains of Myf-5 because for MyoDI (Davis et al., 1990) and other HLH proteins (Murre et al., 1989b; Voronova and Baltimore, 1990) both regions have been shown to be involved in these biochemical
activities. We also generated Myf-5 deletion mutants at the amino-terminus and truncations at the carboxyl-terminus and examined the consequences on Myf-5 activity. All mutant proteins were analysed for their capacity to heterodimerize with the widely expressed bHLH proteins E12/E2-5 and their ability to bind to the E-box consensus sequence of the MLC1/3 enhancer (identical to the MCK E-box) in the presence of E12. Transactivation of a muscle-specific reporter gene was assayed in lOT1/2 cells using a plasmid in which the CAT gene was linked to the MLC enhancer and the thymidine kinase (tk) basal promoter (MCL-CAT). Alternatively, a reporter CAT gene containing only a tetrameric Myf-5 binding site upstream of the tk promoter (MCK 4R-CAT) was also used. Additionally, we examined activation of the endogenous myosin heavy chain (MHC) gene by immunostaining of lOT 1/2 cells following transient transfections with various Myf-5 expressing vectors. Heterodimerization of Myf-5 with E2-5/E12 requires the intact amphipathic HLH domain Protein dimerization was determined in the absence of DNA binding sites. In vitro translated Myf-5 wild-type and mutant proteins at approximately equal concentrations (Figure 1) were incubated with cotranslated E2-5 protein (or E12; data not shown) and subjected to immunoprecipitation using a Myf-5-specific monoclonal antibody (Braun and Arnold, 1991). Precipitated complexes were analysed on SDS -polyacrylamide gels and the extent of heterodimerization was estimated from the amount of coprecipitated E-type HLH protein. DNA binding and transactivation were also measured for each mutant. The first series of mutations was
1845
B.Winter, T.Braun and H.-H.Arnold
intended to substitute amino acid residues in the putative HLH region by prolines in order to disrupt the putative helical structure and to replace hydrophobic by hydrophilic amino acids in positions which were predicted to form the hydrophobic surface of the amphipathic helical barrel. The results obtained with these mutants and E2-5 are illustrated in Figure 1. Proline introduced either in helix I (MM7) or in helix II (MM10) resulted in proteins which completely failed to heterodimerize. Similarly, substitutions of crucial hydrophobic residues as Val98 with Ala in helix I (MM6) and Ile13 1 with Ser in helix II (MM 1 1) led to severe reduction and loss of dimerization, respectively. In contrast, replacing hydrophobic ILe128 by the hydrophobic amino acid Val had no effect (MM 13). Reduction of dimerization was also observed when non-hydrophobic amino acids such as Lysl 19 and Tyrl3O were altered to Met and Val, respectively (MM9; MM 14; MM 15). The strong influence of the positive charge of LysI 19 suggests that dimerization may be stabilized not only by hydrophobic interactions but also by specific salt bridges. The inhibitory effect of Val replacing Tyrl30 was surprising since the same amino acid occupies the corresponding position in the HLH domain of E12/E2-5 which readily homo- and heterodimerizes. It is interesting to note that Tyrl30 in Myf-5 is one of the highly conserved amino acid residues in the helix II of most HLH proteins while Val invariantly occupies the corresponding position in the non-cell-type-specific HLH proteins encoded by the E2A gene and in the Drosophila protein daughterless. This result then shows that the context in which this amino acid is placed strongly affects the capacity to heterodimerize suggesting that not only intramolecular but also intramolecular interactions may play an important role. Slight reduction of heterodimer formation was also observed when Prol 18 defining the N-terminal border of helix II was changed to Leu (MM8) which in effect shortens the intervening loop by extending helix H at its amino-terminus. Although we have not mutagenized the loop structure itself, our result is compatible with the idea that the loop may be responsible for the appropriate positioning of the two helices and therefore may require a minimal length. All mutants that failed to dimerize also did not bind to DNA (Figure IC) and consequently did not activate transcription, confirming that protein-protein interactions are a prerequisite for forming high affinity DNA binding complexes. Taken together, the mutational analysis of the Myf-5 HLH domain showed that both the helical structure and the amphipathic nature of this domain are essential for heterodimerization but in addition other molecular interactions such as salt bridges probably contribute to complex formation. The basic region of Myf-5 specifies DNA binding and affects transcriptional activation of muscle genes Site-directed mutagenesis of the 20 amino acid basic region of Myf-5 was performed in order to analyse the influence of the positively charged amino acids that are arranged in three clusters. We selected primarily basic residues for mutation as it is generally believed that DNA binding is mediated mainly by positively charged amino acids. Some additional mutants were made for controls. Additionally, we tested the importance of two amino acids (Ala88 and Thr89) that have been shown to be critical for muscle-specific transcriptional activation by MyoD 1 and myogenin (Davis et al., 1990; Weintraub et al., 1991b; Brennan et al., 1991).
1846
This motif is invariable in the basic regions of all musclespecific HLH proteins from species as diverse as Caenorhabditis elegans, Drosophila, Xenopus laevis, chicken, mouse and man but differs from the corresponding amino acids in the ubiquitously expressed bHLH protein E12/E47 and E2-2 that do not activate myogenesis. As shown in Figure 2, we obtained a set of mutant proteins that was impaired in DNA binding and consequently also in transcriptional activation of muscle-specific test genes. These mutations mapped to the basic clusters B2 and B3 (MM 1, MM23 and MM5), indicating that both of these regions are important for the interaction with DNA. MM 1 lacking only two of the three basic residues of B2 retained partial binding activity and also weak transactivation. Two prolines introduced between B2 and B3 (MM3) also inactivate DNA binding, suggesting that the DNA binding domain may resume an a-helical structure. In contrast to these inactivating mutations, substitutions of residues outside of clusters B2 and B3 did not affect DNA binding. In particular, the basic amino acids of the conserved cluster B1 could be totally replaced by neutral amino acids (MM22) without reduction of DNA binding or transactivation. This suggests that the cluster Bl is not part of the actual DNA binding domain. Similarly, an alanine/valine substitution in the position immediately downstream of B2 (MM2), showed no or marginal reduction of DNA binding. Replacement of the basic region of MyoD1 and myogenin with the corresponding region of E12 has been shown to result in the complete abolition of activation for muscle-specific test genes without substantially affecting DNA binding (Davis et al., 1990; Brennan et al., 1991). This property depends on a conserved motif within the basic region, referred to as the myogenic recognition motif (MRM). To analyse the importance of this motif in Myf-5, we replaced the conserved amino acids Ala88 and Thr89 with asparagines, the corresponding amino acids of the E12/E47 basic region (MM4). This mutant showed DNA binding similar to wild-type Myf-5 but completely failed to transactivate the MLC -CAT reporter plasmid (Figure 2) and did not induce myogenesis in lOT 1/2 cells (data not shown). This result indicates that, like MyoDl and myogenin, the MRM in the basic region of Myf-5 regulates transcriptional activation distinguishably from DNA binding. All of the mutations in the Myf-5 basic domain showed normal heterodimerization with E12 and E2-5. Inactivating mutations in the basic domain of Myf-5
result in transdominant inhibitors of wild-type Myf-5 As it is believed that myogenic bHLH proteins function in cells as dimeric or oligomeric complexes, inactivating mutations in the basic region should inhibit wild-type Myf-5 activity by competition with the HLH dimerization partner. This mode of action would predict a concentration-dependent dominant negative effect that has been demonstrated for the HLH protein Id which lacks a basic domain (Benezra et al., 1990). To test this notion, pEMSV-Myf5 expressing wildtype protein was cotransfected into lOT1/2 fibroblasts with excess vector plasmid expressing various Myf-5 mutant proteins. Transactivation by Myf-5 was measured on the MCK 4R-CAT reporter plasmid and also on MLC-CAT (data not shown). As illustrated in Figure 3, the non-binding mutant Myf-5 MM5 strongly attenuated Myf-5 dependent CAT activation. Likewise, coexpression of the Myf-5 mutant protein MM4 which binds to DNA but does not activate,
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strongly inhibited wild-type Myf-5 activity, whereas the helix H mutant MM 10, which cannot dimerize, had no effect. The same results were obtained for the activation of the MHC gene in IOT1/2 cells (data not shown). These findings
provide evidence that Myf-5 indeed acts as an oligomeric complex in vivo and mutations that impinge on basic domain functions convert Myf-5 from an activator to an inhibitor of myogenesis. Thus, both hetero-oligomers that cannot bind
1847
B.Winter, T.Braun and H.-H.Arnold
to DNA and inactive complexes which compete with wildtype proteins for DNA binding constitute mechanisms by which myogenesis can be inhibited.
in vitro translation and the accumulation of mutant proteins in transiently transfected COS cells (data not shown). It was difficult to determine the expression levels directly in lOT 1/2 cells due to the generally low concentrations obtained in transient transfections. All amino-terminal Myf-5 deletion mutants have been shown to bind to DNA in vitro as efficiently as the wild-type Myf-5 protein (data not shown). In transient transfection assays, expression of the endogenous myogenic regulatory genes is insignificant relative to the exogenous factor expressed from the transfected plasmid (Chakraborty et al., 1991). Thus the activation by the Myf-5 mutants in our assay should reflect their intrinsic activity. As demonstrated in Figure 4, Myf-5 mutant N-1, which lacks amino acids 14-30, was unexpectedly more effective in transactivation than the wild-type protein, suggesting a negative control function in this region. Alternatively, this mutant protein may resume a favourable conformation in its transactivator domain. Processive larger deletions spanning amino acids 14-47 and 1-50 in the mutants N-3 and N-14/II respectively, resulted in marked reduction of transactivation. Virtually no activity was observed for the mutant N-10, which lacks amino acids 14-74; this provides evidence that a critical element for the biological activity of Myf-5 is located in the amino-terminus, particularly between amino acids 48 and 75. This region encompasses the conserved sequence present in all myogenic HLH
Transcriptional activation by Myf-5 requires two regions located outside of the bHLH domain To define regions of the Myf-5 protein required for muscle gene activation, amino- and carboxyl-terminal Myf-5 deletion mutants were tested for their ability to initiate the myogenic program in transiently transfected IOTl/2 fibroblasts as assayed by the number of MHC positive cells. Transcriptional activation was also determined in transiently transfected lOT 1/2 cells using the MCK 4R-CAT (data not shown) or the MLC -CAT plasmids as reporter genes. To provide the appropriate ribosome binding site and the translation start signal for the expression of amino-terminal deletions, the 5' leader and the sequence encoding the first 13 amino acids of Myf-5 were generally maintained in most mutant constructs. In the mutant N-14/II lacking 50 aminoterminal amino acids, the Myf-5 leader was replaced by the 5' upstream sequence of the /-globin mRNA linked to the truncated Myf-5 mRNA by an NcoI linker which supplies the initiation codon AUG in-frame. The carboxy-terminal truncations were generated by limited digestions with exonuclease III followed by the addition of translational stop signals. Proper expression of all constructs was ensured by
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Muscle-specific transactivation by the bHLH protein Myf-5
proteins which was originally designated as the cysteine-rich domain (Davis et al., 1987). Successive deletions of the carboxyl-terminus resulted in mutants C-3 (lacking amino acids 216-255), C-2 (lacking amino acids 198-255) and C-I (which lacks amino acids 150 -255). The carboxyl-terminal truncations revealed that Myf-5 contains an element located downstream of the bHLH domain that is also essential for transcriptional activation. This observation is in agreement with our previous study in which we demonstrated that the C-terminal half of the Myf-5 molecule can confer transactivation on the GAL4 DNA binding domain (Braun et al., 1990b). Because the mutant C- 1, which contains a truncation shortly behind helix [1, was totally inactive whereas mutants C-2 and C-3 retained partial but not full activity, we conclude that the complete carboxyl-terminal segment between amino acids 149 and 255 constitutes a transcriptional activator domain. It is interesting to note that the shorter deletion protein C-2 was consistently more active than C-3 which contains one complete block and the major part of a second block of hydroxylated amino acids. These sequences are highly conserved in all myogenic HLH proteins and it has therefore been hypothesized that they are of functional importance (Braun et al., 1989b). Deletion of one of these regions in the mutants C-2 and C-S or deletion of both in the mutant C4 apparently results in dramatically enhanced activity. This result indicates that both conserved sequence elements are not only dispensable for transactivation but rather exert a negative effect. As mentioned previously, mutation of the 'myogenic recognition motif' (Ala88/Thr89) in the basic region of Myf-5 (Myf-5 MM4) resulted in complete inactivation, indicating that both transactivator domains of Myf-5 are subject to regulation by the basic region. In summary then, deletion analysis of the Myf-5 protein revealed that two regions located upstream and downstream of the bHLH domain are essential for transcriptional activation of muscle-specific genes. Apparently both of these elements need to be present in the molecule since deletion of either one resulted in total inactivation, indicating that they cannot function independently of each other in the context of the native Myf-5 protein. In addition, the correct basic region is required for muscle-specific transactivation since the mutant Myf-5 MM4, with two asparagines replacing the invariant Ala88/Thr89 sequence, fails to activate, although, as we have shown before, this mutant binds to DNA normally. While the amino-terminal activator region contains a sequence that is fairly conserved in all muscle regulatory HLH proteins, the carboxyl-terminal sequences have completely diverged and show no similarity to other known transactivator domains (for Myf-5 sequence see Braun et al., 1989a). Both Myf-5 regions required for transactivation confer activity on the heterologous GAL4 DNA binding domain We have previously reported that chimeric GAL4-Myf5 proteins activate transcription of a GAL4 reporter gene in lOT1/2 fibroblasts (Braun et al., 1990b). Complementary to the deletion analysis described here and in order to map the activation domains of Myf-5 more precisely, we extended our investigations and constructed GAL4 hybrid molecules by fusing a variety of Myf-5 deletion mutants to the GAL4 (1-147) DNA binding domain. To control the expression of the various constructs driven by the constitutive SV40
promoter (Martin et al., 1990), we analysed nuclear extracts of COS cells following transient transfections with plasmids expressing the GAL4 - MyfS chimeric proteins. Western blots were immunostained with GALA-specific antiserum (kindly provided by M.Ptashne) which showed that the different hybrid proteins accumulated to comparable levels (Figure 5). Transcriptional activation was assayed following transient transfections of lOT1/2 fibroblasts with the various expression plasmids and the reporter plasmid (G5EIbCAT containing a pentameric GAL4 DNA binding site upstream of the Elb TATA box (Martin et al., 1990). We consistently found that this reporter construct showed lower unspecific background activation than the one used in our previous report (Braun et al., 1990b). Activation with GAL4 - Myf5 fusion proteins was measured relative to the control level obtained with GALA (1-147) alone. A control reporter plasmid lacking GAL4 binding sites gave no activity. As shown in Figure 6, GAL-MyfS (135-255) carrying the carboxyl-terminal half of Myf-5 activated 50-fold relative to GAL4 (1 - 147) which identifies a strong activator domain and confirms our previous results (Braun et al., 1990b). GAL-Myf5del46-135, which lacks the cysteine-rich region and the bHLH domain, was -5-fold more active than the carboxy-terminus alone suggesting that the amino- and carboxy-termini co-operate in the activation. The Myf-5 bHLH region fused to GAL4 (GAL-MyfSbHLH) failed to activate transcription, indicating that this region by itself has no transactivating function. Plasmid M-1, which encodes 46 amino acids of the amino-terminus, showed only a marginal activation, whereas M-2, which expresses amino acids 1 -76, consistently activated 10- to 20-fold. Interestingly, M-2a (containing the amino-terminus including part of the basic region) was inactive. These results confirm the existence of two separate transactivator regions located upstream and downstream of the bHLH domain of Myf-5. The two transactivators, apparently function synergistically when combined. It should be mentioned that e I
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L Fig. 6. Transactivation of the GAL5Elb-CAT reporter plasmid by GAL4-Myf5 chimeric proteins in IOTI/2 fibroblasts. Expression vectors for GAL4-Myf5 chimeras were constructed as described in Materials and methods. CAT activity was deterrmined following transient transfections of IOTI/2 fibroblasts with 2 jig of transactivator and 2 /Ag of reporter plasmid. Numbers indicate CAT activities relative to that obtained with the GAL4 (1-147) plasmid encoding only the DNA binding domain.
GAL4 -Myf5 constructs that included the complete bHLH domain were less active and did not show this strong co-operativity (see also Figure 7). Consistent with and complementary to the results obtained for the Myf-5 deletion mutants C-2 to C-5, the GAL4 chimeras M-3 to M-6, encoding amino acids 1-46 fused to various carboxy-terminal regions, exhibited high transactivation. The constructs M-5 and M-6, which lacked all or part of the conserved segments of hydroxylated amino acids, were most active. M-3 and M4 (lacking 40 and 58 carboxy-terminal amino acids respectively) showed lower activity than the complete C-terminal half of Myf-S (GAL-MyfSdel46-135). The construct M-7, which expresses amino acids 1-46 and 135-149, showed virtually 1850
no transactivation. Taken together, these results indicate that the carboxy-terminal transactivator domain is located in a segment spanning amino acids 150-255 excluding the serine/threonine-rich boxes. It is interesting to note that bisection of the region encompassing amino acids 135 -255 apparently inactivates the carboxyl-terminal transactivator domain [compare M-8, M-9 and GAL-Myf5 (135 -255)], although these fragments retain activity, albeit at a reduced level, when combined with the amino-terminal 46 amino acids (see M4). This observation once more suggests that the carboxyl-terminal activator domain requires co-operative interactions with the amino-terminus. It may also indicate that two regions within the amino-terminus, one proximal and one distal, also co-operate for maximal activity.
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