title: Homodimeric MyoD preferentially binds tetraplex structures of regulatory sequences of muscle-specific genes.
abstract: 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.
authors.affiliation: Department of Biochemistry, Rappaport Faculty of Medicine, Technion-Israel Institute of Technology, P. O. Box 9649 Bat Galim, Haifa 31096, Israel.
authors.forename: Shulamit
authors.initials: S
authors.lastname: Etzioni
authors.forename: Anat
authors.initials: A
authors.lastname: Yafe
authors.forename: Samer
authors.initials: S
authors.lastname: Khateb
authors.forename: Pnina
authors.initials: P
authors.lastname: Weisman-Shomer
authors.forename: Eyal
authors.initials: E
authors.lastname: Bengal
authors.forename: Michael
authors.initials: M
authors.lastname: Fry
mesh_terms: D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins; D004247:DNA / Q000737:chemistry* / Q000378:metabolism; D004268:DNA-Binding Proteins; D019281:Dimerization; D024721:E-Box Elements; D054856:G-Quadruplexes; D006801:Humans; D009124:Muscle Proteins / Q000235:genetics*; D017570:MyoD Protein / Q000737:chemistry / Q000378:metabolism*; D009690:Nucleic Acid Conformation; D011485:Protein Binding; D014157:Transcription Factors; D014158:Transcription, Genetic; D015533:Transcriptional Activation
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D004247:DNA; D004268:DNA-Binding Proteins; D009124:Muscle Proteins; D017570:MyoD Protein; D014157:Transcription Factors