title: Formation of triple-stranded DNA at d(GA.TC)n sequences prevents nucleosome assembly and is hindered by nucleosomes.
abstract: Simple repeating d(GA.TC)n DNA sequences are frequently found at eukaryotic promoters, and in some cases they have been shown to be nucleosome free in vivo. These sequences show a high degree of structural polymorphism and are capable of adopting several types of non-B-DNA conformations. Here we show that the structural versatility of these sequences affects their ability to be packed into nucleosomes. Nucleosome assembly onto short double-stranded DNA fragments carrying d(GA.TC)n sequences of different length (n = 10 and n = 22) is very efficient. However, when the simple repeating sequence is forming a [CT(GA.TC)] triplex, nucleosome assembly is either prevented, as in the case of the d(GA.TC)22 sequence, or results in the destabilization of the triple-stranded conformation, as in the case of the d(GA.TC)10 sequence. Similarly, formation of triple-stranded DNA is hindered when the sequence is organized as nucleosomes. Efficient formation of triplex DNA occurs only at relatively high ionic strength (0.6 M NaCl), when the nucleosome is partially destabilized, and results in the disruption of the nucleosomal particle. These results indicate that nucleosome assembly and triplex formation are competing processes.
authors.affiliation: Department de Biologia Molecular i Cel.lular, Centre d'Investigació i Desenvolupament, Consejo Superior de Investigaciones Científicas, Jordi Girona Salgado 18-26, 08034 Barcelona, Spain. fambmc@cid.csic.es
authors.forename: M L
authors.initials: ML
authors.lastname: Espinás
authors.forename: E
authors.initials: E
authors.lastname: Jiménez-García
authors.forename: A
authors.initials: A
authors.lastname: Martínez-Balbás
authors.forename: F
authors.initials: F
authors.lastname: Azorín
mesh_terms: D001483:Base Sequence; D004247:DNA / Q000378:metabolism*; D003850:Deoxyribonuclease I / Q000378:metabolism; D008969:Molecular Sequence Data; D009690:Nucleic Acid Conformation; D009707:Nucleosomes / Q000378:metabolism*; D009994:Osmolar Concentration
chemical_list: D009707:Nucleosomes; C051802:triplex DNA; D004247:DNA; D003850:Deoxyribonuclease I