Homodimeric MyoD Preferentially Binds Tetraplex Structures of Regulatory Sequences of Muscle-specific Genes

Myogenic transcription is activated by the binding of heterodimers of the basic helix-loop-helix proteins MyoD and E12 or E47 to a consensus E-box sequence, d(CANNTG), in promoter or enhancer regions of muscle-specific genes. Homodimers of MyoD bind E-box less tightly and are less efficient activators of transcription. Recent results from our laboratory (Yafe, A., Etzioni, S., Weisman-Shomer, P., and Fry, M. (2005) Nucleic Acids Res. 33, 2887–2900) indicate that regulatory sequences of several muscle-specific genes contain a disproportionate high content of guanine clusters that readily form hairpin and parallel-stranded unimolecular and bimolecular tetraplex structures. Here we have shown that homodimers of full-length recombinant MyoD formed complexes with bimolecular tetraplex structures of muscle-specific regulatory sequences but not with their double-stranded, hairpin, or unimolecular tetraplex forms. Preferential binding of homodimeric MyoD to bimolecular tetraplex DNA structures over E-box DNA was reflected by the 18.7–39.9-fold lower dissociation constants, Kd, of the MyoD-tetraplex DNA complexes. Conversely, MyoD-E47 heterodimers formed tighter complexes with E-box as indicated by their 6.8–19.0-fold lower Kd relative to complexes with bimolecular tetraplex DNA structures. Similarly, homodimers of the 60-amino acid basic helix-loop-helix domain of MyoD bound E-box more efficiently and tetraplex DNA less efficiently than homodimers of full-length MyoD. It might be that the favored binding of MyoD homodimers to tetraplex DNA structures lowers their ability to activate muscle-specific gene transcription, whereas the formation of MyoD-E47 heterodimers and their preferential binding to E-box DNA enhance transcription. Myogenic transcription is activated by the binding of heterodimers of the basic helix-loop-helix proteins MyoD and E12 or E47 to a consensus E-box sequence, d(CANNTG), in promoter or enhancer regions of muscle-specific genes. Homodimers of MyoD bind E-box less tightly and are less efficient activators of transcription. Recent results from our laboratory (Yafe, A., Etzioni, S., Weisman-Shomer, P., and Fry, M. (2005) Nucleic Acids Res. 33, 2887–2900) indicate that regulatory sequences of several muscle-specific genes contain a disproportionate high content of guanine clusters that readily form hairpin and parallel-stranded unimolecular and bimolecular tetraplex structures. Here we have shown that homodimers of full-length recombinant MyoD formed complexes with bimolecular tetraplex structures of muscle-specific regulatory sequences but not with their double-stranded, hairpin, or unimolecular tetraplex forms. Preferential binding of homodimeric MyoD to bimolecular tetraplex DNA structures over E-box DNA was reflected by the 18.7–39.9-fold lower dissociation constants, Kd, of the MyoD-tetraplex DNA complexes. Conversely, MyoD-E47 heterodimers formed tighter complexes with E-box as indicated by their 6.8–19.0-fold lower Kd relative to complexes with bimolecular tetraplex DNA structures. Similarly, homodimers of the 60-amino acid basic helix-loop-helix domain of MyoD bound E-box more efficiently and tetraplex DNA less efficiently than homodimers of full-length MyoD. It might be that the favored binding of MyoD homodimers to tetraplex DNA structures lowers their ability to activate muscle-specific gene transcription, whereas the formation of MyoD-E47 heterodimers and their preferential binding to E-box DNA enhance transcription. The development of skeletal muscle from pluripotent mesodermal stem cells involves multiple consecutive steps. At first, cells commit to myogenic precursors and proliferate as myoblasts. In a following differentiation step, myoblasts cease to divide, begin to express muscle-specific genes, and finally fuse to form fully differentiated syncytial myotubes (1Tajbakhsh S. Curr. Opin. Genet. Dev. 2003; 13: 413-422Crossref PubMed Scopus (68) Google Scholar, 2Schnorrer F. Dickson B.J. Dev. Cell. 2004; 7: 9-20Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar). Four myogenic regulatory factors (MRFs), 1The abbreviations used are: MRF, myogenic regulatory factors; bHLH, basic helix-loop-helix; sMtCK, sarcomeric mitochondrial creatine kinase. MyoD, Myf-5, myogenin, and MRF4, regulate the coordinated activation of multiple muscle-specific genes during myogenesis. These transcription factors comprise a subgroup within the superfamily of basic helix-loop-helix (bHLH) proteins (for review see Ref. 3Sabourin L.A. Rudnicki M.A. Clin. Genet. 2000; 57: 16-25Crossref PubMed Scopus (565) Google Scholar). Targeted inactivation of the various MRFs in the germ line of mice indicated that the commitment of proliferating somitic cells to the myogenic lineage is controlled by MyoD and Myf-5 (4Braun T. Rudnicki M.A. Arnold H.H. Jaenisch R. Cell. 1992; 71: 369-382Abstract Full Text PDF PubMed Scopus (559) Google Scholar, 5Rudnicki M.A. Braun T. Hinuma S. Jaenisch R. Cell. 1992; 71: 383-390Abstract Full Text PDF PubMed Scopus (765) Google Scholar, 6Rudnicki M.A. Schnegelsberg P.N. Stead R.H. Braun T. Arnold H.H. Jaenisch R. Cell. 1993; 75: 1351-1359Abstract Full Text PDF PubMed Scopus (1335) Google Scholar), whereas the subsequent differentiation of committed myoblasts into myocytes and myotubes requires the action of myogenin and MRF4 (7Zhang W. Behringer R.R. Olson E.N. Genes Dev. 1995; 9: 1388-1399Crossref PubMed Scopus (249) Google Scholar, 8Rawls A. Morris J.H. Rudnicki M. Braun T. Arnold H.H. Klein W.H. Olson E.N. Dev. Biol. 1995; 172: 37-50Crossref PubMed Scopus (113) Google Scholar, 9Hasty P. Bradley A. Morris J.H. Edmondson D.G. Venuti J.M. Olson E.N. Klein W.H. Nature. 1993; 364: 501-506Crossref PubMed Scopus (1039) Google Scholar, 10Nabeshima Y. Hanaoka K. Hayasaka M. Esumi E. Li S. Nonaka I. Nature. 1993; 364: 532-535Crossref PubMed Scopus (732) Google Scholar). The HLH section of the bHLH domain is responsible for oligomerization of MRF proteins, whereas its basic region is required for their specific binding to DNA (11Davis R.L. Cheng P.F. Lassar A.B. Weintraub H. Cell. 1990; 60: 733-746Abstract Full Text PDF PubMed Scopus (528) Google Scholar). Heterodimers of MyoD with the bHLH proteins E12, E47, and ITF1 were found to be formed at a greater efficiency than homodimers of MyoD (Refs. 11Davis R.L. Cheng P.F. Lassar A.B. Weintraub H. Cell. 1990; 60: 733-746Abstract Full Text PDF PubMed Scopus (528) Google Scholar,12Murre C. McCaw P.S. Vaessin H. Caudy M. Jan L.Y. Jan Y.N. Cabrera C.V. Buskin J.N. Hauschka S.D. Lassar A.B. Weintraub H. Baltimore D. Cell. 1989; 58: 537-544Abstract Full Text PDF PubMed Scopus (1303) Google Scholar, reviewed in Ref. 13Rudnicki M.A. Jaenisch R. BioEssays. 1995; 17: 203-209Crossref PubMed Scopus (379) Google Scholar). Studies of differentiation in vitro revealed that heterodimers of MyoD with E12 or E47 proteins activated transcription by their binding to a conserved E-box motif, d(CANNTG), in the promoter or enhancer regions of muscle-specific genes. Also, the affinity of MyoD-E12 heterodimers for E-box was significantly higher than that of their respective homodimers (12Murre C. McCaw P.S. Vaessin H. Caudy M. Jan L.Y. Jan Y.N. Cabrera C.V. Buskin J.N. Hauschka S.D. Lassar A.B. Weintraub H. Baltimore D. Cell. 1989; 58: 537-544Abstract Full Text PDF PubMed Scopus (1303) Google Scholar, 14Lassar A.B. Davis R.L. Wright W.E. Kadesch T. Murre C. Voronova A. Baltimore D. Weintraub H. Cell. 1991; 66: 305-315Abstract Full Text PDF PubMed Scopus (688) Google Scholar). Homodimers of the 60 amino-acid-long protein fragment that spans just the bHLH domain of MyoD were shown to be sufficient for specific DNA binding in vitro (12Murre C. McCaw P.S. Vaessin H. Caudy M. Jan L.Y. Jan Y.N. Cabrera C.V. Buskin J.N. Hauschka S.D. Lassar A.B. Weintraub H. Baltimore D. Cell. 1989; 58: 537-544Abstract Full Text PDF PubMed Scopus (1303) Google Scholar), and similar to full-length MyoD, they were capable of inducing myogenesis in stably transfected mouse fibroblasts (15Tapscott S.J. Davis R.L. Thayer M.J. Cheng P.F. Weintraub H. Lassar A.B. Science. 1988; 242: 405-411Crossref PubMed Scopus (595) Google Scholar). As originally proposed by Larsen and Weintraub (16Larsen A. Weintraub H. Cell. 1982; 29: 609-622Abstract Full Text PDF PubMed Scopus (322) Google Scholar), some regulatory proteins may affect transcription by specifically recognizing altered DNA conformation rather than a nucleotide sequence in B-DNA. In line with this idea, the ability of myogenic proteins to recognize DNA conformations other than double strands was first inferred from the sequence-specific binding of MyoD and of a muscle protein designated MF3 to single-stranded E-box motif (17Santoro I.M. Yi T.M. Walsh K. Mol. Cell. Biol. 1991; 11: 1944-1953Crossref PubMed Scopus (55) Google Scholar). More significantly, recombinant MyoD was reported to form complexes with tetrahelical structures of a guanine-rich mouse creatine kinase enhancer sequence or of Tetrahymena telomeric DNA (18Walsh K. Gualberto A. J. Biol. Chem. 1992; 267: 13714-13718Abstract Full Text PDF PubMed Google Scholar). Measurements of dissociation constants indicated that the binding of MyoD to tetraplex DNA structures is 4–5-fold tighter than to E-box DNA (18Walsh K. Gualberto A. J. Biol. Chem. 1992; 267: 13714-13718Abstract Full Text PDF PubMed Google Scholar). We reported recently that promoter and enhancer regions of several muscle-specific genes contained segments with a disproportional high frequency of clusters of contiguous guanine residues (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar). Interestingly, guanine-rich DNA tracts in the regulatory regions of the muscle-specific genes readily folded into a variety of secondary structures, hairpin and parallel-stranded unimolecular and bimolecular tetraplexes (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar). To examine whether these secondary structures may be of potential regulatory significance, we examined in this work the association of MyoD homodimers and MyoD-E47 heterodimers with E-box DNA and with the various secondary structures of guanine-rich muscle gene regulatory sequences. We report that MyoD homodimers specifically bound bimolecular DNA tetraplexes but not single or double strands or hairpin or monomolecular tetraplex structures of the guanine-rich muscle gene DNA sequences. Further, measurements of dissociation constants of complexes of MyoD homodimers with DNA indicated that complexes of this protein with the bimolecular tetraplexes were more stable than its complex with E-box DNA. Conversely, MyoD-E47 heterodimers bound E-box DNA more tightly than bimolecular tetraplex DNA structures. We speculate that the preferential binding of the relatively inactive MyoD homodimers or the transcriptionally active MyoD-E47 heterodimers to tetraplex or E-box may to the of muscle-specific gene during myogenesis. of and and DNA nucleotide sequences of the DNA that are in were from guanine-rich promoter regions of the genes sarcomeric mitochondrial creatine kinase or and Ref. A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google or contained the E-box The were by in M. L.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). The single-stranded were at their by in and monomolecular and bimolecular tetraplex structures of the DNA were formed as we (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar). DNA was by P. Y. Fry M. J. Biol. Chem. 2000; Full Text Full Text PDF PubMed Scopus Google of single-stranded used in this in a and of and of recombinant of E47 were as Baltimore D. Cell. 1991; Full Text PDF PubMed Scopus Google Scholar). of that full-length MyoD protein or its bHLH domain of in E. P.N. A. Weintraub H. I.M. S. A. PubMed Scopus Google were that contained at their respective or or sequences. The were and to and The recombinant were into and the of was by of their nucleotide sequences. To recombinant MyoD protein and its bHLH MyoD or bHLH were into E. the were to of in and and of the proteins was by to for The recombinant proteins were to from the by affinity The was by of protein for and at with of of to DNA and of of the of MyoD or its bHLH domain or heterodimers were formed to their binding to the various DNA by for at of recombinant MyoD or bHLH or of MyoD and in that in a of and of for binding in a of of MyoD or bHLH homodimers or heterodimers and DNA in and of for the binding of E-box DNA or of the examined guanine-rich myogenic sequences contained of for the binding of bimolecular tetraplex DNA structures of muscle-specific regulatory sequences contained of single-stranded of the The were for at complexes were from DNA by at and in in in of the DNA was a into the In a that required greater of was the into a The were and the relative of of and DNA structures were by To of dissociation constants, Kd, of complexes of MyoD or MyoD-E47 with E-box DNA or with bimolecular tetraplex structures of guanine-rich muscle-specific DNA of DNA were with a of protein as of the complexes from their relative were by of the Kd were from the of the of a of the results as P. Fry M. Res. PubMed Scopus Google MyoD and E-box and DNA in and The HLH of MyoD or proteins formation of homodimers of protein or their association to form We first whether the binding of E-box DNA and bimolecular tetraplex of DNA is by the or of these of MyoD and was by protein or in the of a protein The proteins were DNA binding with E-box DNA or bimolecular tetraplex DNA. complexes were from DNA by results of this are shown in These indicated that MyoD and E47 formed complexes with the of DNA. The complexes in the more than the as for the MyoD as with the Baltimore D. Cell. 1991; Full Text PDF PubMed Scopus Google Scholar). of of DNA with the of MyoD and complexes that the and the more complexes of MyoD and The of DNA complexes with the of MyoD and that heterodimers of the and MyoD and were responsible for the binding of DNA. with the of these the or more of MyoD or was with a homodimeric of MyoD of that guanine-rich tracts from enhancer or promoter sequences of muscle-specific genes readily folded into hairpin or parallel-stranded unimolecular or bimolecular tetraplex structures (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar). Also, as was MyoD with tetraplex of sequences of mouse creatine kinase enhancer (18Walsh K. Gualberto A. J. Biol. Chem. 1992; 267: 13714-13718Abstract Full Text PDF PubMed Google or promoter (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google more efficiently than with E-box DNA. MyoD the transcription of muscle-specific genes regulatory sequences formed secondary structures, we the binding of homodimeric MyoD to double-stranded, hairpin, or tetraplex structures of these guanine-rich of MyoD homodimers were with a of bimolecular tetraplex structures of the or DNA or with the that these formed with their respective sequences for As in MyoD with the bimolecular tetraplex structures of whereas its binding to the of sequences was As was shown (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar), the sequence formed a of unimolecular and bimolecular parallel-stranded in the of indicated whereas the MyoD homodimers bound bimolecular tetraplex DNA they not with the unimolecular tetraplex form of this In other we found that in to the efficient association of MyoD with bimolecular tetraplex DNA to form complexes with hairpin structures of the sequence (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar). MyoD in its homodimeric to bind bimolecular tetraplex structures of guanine-rich tracts of promoter sequences of the muscle-specific and genes. of MyoD whether the of the bimolecular tetraplex of a sequence of the muscle-specific gene was by its association with MyoD. tetraplex structures of the DNA were bound to homodimeric recombinant MyoD to form whereas not contain MyoD. of the binding or were for at or were at for various of DNA was by of the to the DNA was of bound protein by and bimolecular tetraplex DNA was from single-stranded DNA by shown in indicated that the association with MyoD the of bimolecular tetraplex DNA. In with (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar), results shown in indicated that bimolecular tetraplex DNA was with of its complex with MyoD was the of the tetraplex DNA was significantly that of its was at whereas of the bimolecular tetraplex DNA was at of the tetraplex DNA DNA the for MyoD, whereas E-box DNA by bHLH to full-length MyoD (12Murre C. McCaw P.S. Vaessin H. Caudy M. Jan L.Y. Jan Y.N. Cabrera C.V. Buskin J.N. Hauschka S.D. Lassar A.B. Weintraub H. Baltimore D. Cell. 1989; 58: 537-544Abstract Full Text PDF PubMed Scopus (1303) Google Scholar, Weintraub H. Science. 1990; PubMed Scopus Google Scholar), its bHLH domain is capable of the muscle differentiation by in a sequence-specific with E-box DNA motif, (11Davis R.L. Cheng P.F. Lassar A.B. Weintraub H. Cell. 1990; 60: 733-746Abstract Full Text PDF PubMed Scopus (528) Google Scholar, S.J. Davis R.L. Thayer M.J. Cheng P.F. Weintraub H. Lassar A.B. Science. 1988; 242: 405-411Crossref PubMed Scopus (595) Google Scholar). In of the of MyoD to bind tetraplex structures of muscle-specific DNA we the relative binding of homodimers of full-length MyoD or its bHLH domain to bimolecular DNA and E-box DNA. results shown in indicated that the full-length MyoD protein formed complexes with tetraplex DNA in over E-box DNA. the binding full-length MyoD bound bimolecular tetraplex but E-box DNA. The bHLH with E-box DNA more than with the tetraplex DNA. As in bHLH bound E-box DNA and bimolecular tetraplex DNA. relative to full-length MyoD, bHLH bound E-box DNA at greater whereas its to bind the bimolecular tetraplex DNA was by to The preferential to bimolecular tetraplex DNA that full-length MyoD was by the bHLH of its affinity for E-box DNA. MyoD MyoD-E47 Heterodimers E-box DNA over of MyoD are formed at a lower efficiency than its heterodimers with the bHLH proteins E12 or E47 (12Murre C. McCaw P.S. Vaessin H. Caudy M. Jan L.Y. Jan Y.N. Cabrera C.V. Buskin J.N. Hauschka S.D. Lassar A.B. Weintraub H. Baltimore D. Cell. 1989; 58: 537-544Abstract Full Text PDF PubMed Scopus (1303) Google Scholar). Further, transcription of muscle-specific genes is more by MyoD-E47 heterodimers than by MyoD homodimers A.B. Davis R.L. Wright W.E. Kadesch T. Murre C. Voronova A. Baltimore D. Weintraub H. Cell. 1991; 66: 305-315Abstract Full Text PDF PubMed Scopus (688) Google Scholar, E. P.N. A. Weintraub H. I.M. S. A. PubMed Scopus Google Scholar). We the of MyoD homodimers or heterodimers for E-box DNA and for bimolecular tetraplex structures of promoter sequences of muscle-specific genes. To this we of dissociation constants, Kd, of complexes of MyoD homodimers or heterodimers with the various DNA structures. results shown in the of MyoD homodimers or heterodimers for a bimolecular tetraplex DNA The homodimeric MyoD a significantly higher affinity for bimolecular tetraplex DNA than the was reflected by a higher Kd of the complex of the tetraplex DNA with relative to MyoD and In to the preferential binding of MyoD homodimers to tetraplex DNA and results shown in indicated that heterodimers a significantly higher affinity for E-box DNA than for bimolecular tetraplex DNA. As in this the Kd of the complex with E-box DNA was lower than the Kd of its complex with the bimolecular tetraplex of DNA. The of MyoD homodimers and heterodimers for tetraplex DNA structures and E-box DNA was in measurements of Kd of the respective complexes. of these in that the dissociation of the complex of E-box DNA with MyoD homodimers was higher than the Kd of its complex with Conversely, MyoD homodimers bound bimolecular tetraplex DNA structures more tightly than E-box DNA. the Kd of complexes of MyoD homodimers with bimolecular tetraplex structures of the sMtCK, and DNA sequences were lower than dissociation constants of the respective complexes with heterodimers In these results indicated that MyoD homodimers with bimolecular tetraplex DNA structures of muscle-specific regulatory whereas E-box DNA was a for the binding of heterodimers bind E-box DNA more tightly than a bimolecular DNA tetraplex heterodimers that were as were DNA binding with the indicated of E-box DNA or a bimolecular tetraplex of DNA. The complexes were from DNA by and their relative were by as in the to of complexes of E-box DNA with As the complex of with E-box DNA complexes of or MyoD homodimers with this DNA. of of results shown in A. The Kd of the complex was as in of complexes of E-box DNA with The complex with the E-box DNA complexes of or MyoD homodimers with this DNA. of of results in C. The Kd was as in constants of complexes of MyoD homodimers and heterodimers with E-box DNA and with tetraplex structures of regulatory sequences of muscle-specific tetraplex tetraplex tetraplex in a proteins from with tetraplex structures of DNA or proteins bind tetraplex DNA and at high affinity P. Fry M. J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar, W. J. Biol. Chem. 1995; Full Text Full Text PDF PubMed Scopus Google Scholar, R. Fry M. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, T. E. Nucleic Acids Res. PubMed Scopus Google Scholar, P. H. T. L.A. Fry M. J. Biol. 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Gualberto A. J. Biol. Chem. 1992; 267: 13714-13718Abstract Full Text PDF PubMed Google Scholar). In the higher affinity of MyoD for tetrahelical DNA than for its E-box sequence (18Walsh K. Gualberto A. J. Biol. Chem. 1992; 267: 13714-13718Abstract Full Text PDF PubMed Google the that tetraplex structures in DNA may to the of the transcription of muscle-specific genes. of tetraplex DNA structures in gene transcription is not that a tetrahelical DNA domain formed to the promoter of its A. S. A. PubMed Scopus Google Scholar, H. S. Mol. Google Scholar, J. Chem. 2004; PubMed Scopus Google Scholar, T. D. R. E. P. Res. 2004; PubMed Scopus Google Scholar, D. M. S. A. 2004; PubMed Scopus Google Scholar). Conversely, tetraplex structures in the region to enhance transcription of this gene R. K. Nucleic Acids Res. 1992; PubMed Scopus Google Scholar, A. S. A. 2000; PubMed Scopus Google Scholar). Recent results from our laboratory that promoter or enhancer regions of several muscle-specific genes tracts in guanine clusters that readily folded in vitro into hairpin and parallel-stranded unimolecular and bimolecular tetraplex structures (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar). of the binding of homodimers of recombinant MyoD to the secondary structures of these guanine-rich sequences revealed its for association with bimolecular tetraplex DNA structures. for homodimeric MyoD efficiently bound bimolecular tetraplex of the or DNA whereas their structures not form complexes with this protein Similarly, unimolecular tetraplex DNA hairpin DNA (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google formed complexes with the MyoD The association of MyoD with bimolecular DNA was by the in the of the the binding of MyoD homodimers to bimolecular tetraplex DNA was tighter than to the E-box DNA in and in indicated that the dissociation constants of complexes of homodimeric MyoD with bimolecular tetraplex structures of the and DNA sequences were 18.7–39.9-fold lower than the Kd of its complex with E-box DNA. heterodimers of MyoD with a relative affinity to tetraplex and E-box DNA and shown in and indicate that heterodimers bound E-box more tightly than MyoD The of a of in a lower Kd of complexes of E-box DNA with the heterodimers as with complexes with MyoD Further, formed tighter complexes with E-box DNA than with bimolecular tetraplex structures of the muscle-specific gene sequences. the dissociation of the complex of with E-box DNA was 6.8–19.0-fold lower than Kd of its complexes with the various tetraplex DNA structures and Interestingly, the relative binding of of the bHLH domain of MyoD to the DNA structures was more similar to that of the than to homodimeric MyoD. in indicate that E-box DNA was bound by bHLH at higher efficiency than by full-length MyoD. Conversely, bHLH bound bimolecular tetraplex DNA at lower than the full-length The in vitro bimolecular tetraplex structures of the and DNA in as sequence is by a single in the tetraplex structures be formed by the of hairpin structures of guanine-rich sequences in single-stranded DNA. The of a was by the of a bimolecular tetraplex the and promoter sequences that are from in the (19Yafe A. Etzioni S. Weissman-Shomer P. Fry M. Nucleic Acids Res. 2005; 33: 2887-2900Crossref PubMed Scopus (51) Google Scholar). tetraplex structures may be formed in sequences in single-stranded of regulatory regions of DNA. tetraplex structures are in in regulatory regions of muscle-specific genes, they in the of homodimers of MyoD were found to be activators of transcription, binding of MyoD heterodimers with E12 or E47 proteins to the E-box motif is the of muscle-specific gene transcription A.B. Davis R.L. Wright W.E. Kadesch T. Murre C. Voronova A. Baltimore D. Weintraub H. Cell. 1991; 66: 305-315Abstract Full Text PDF PubMed Scopus (688) Google Scholar, E. P.N. A. Weintraub H. I.M. S. A. PubMed Scopus Google Scholar). of MyoD homodimers with its heterodimers E-box the gene activation during muscle of homodimeric MyoD by tetraplex DNA structures to its association with the E-box and or of muscle-specific genes. that the association of MyoD in myoblasts with was with the of the myogenin gene A. S. A. 2003; PubMed Scopus Google Scholar). It be that MyoD homodimers bind to tetraplex DNA in complex with association the ability of homodimeric MyoD to activate transcription. heterodimers of MyoD and protein are the association of the protein with the tetraplex DNA is and the protein is to bind tightly to and to activation of gene transcription. The formation of the heterodimers to the of and to of to the myogenic It is to speculate that the of homodimeric MyoD from its complex with tetraplex DNA might be by DNA M. L.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, H. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google or by DNA proteins P. E. Fry M. Nucleic Acids Res. PubMed Scopus Google Scholar, S. P. I. L.A. Fry M. Nucleic Acids Res. 2004; PubMed Scopus Google that tetrahelical DNA.