title: Structural basis for the identification of an i-motif tetraplex core with a parallel-duplex junction as a structural motif in CCG triplet repeats.
abstract: CCG triplet repeats can fold into tetraplex structures, which are associated with the expansion of (CCG)n trinucleotide sequences in certain neurological diseases. These structures are stabilized by intertwining i-motifs. However, the structural basis for tetraplex i-motif formation in CCG triplet repeats remains largely unknown. We report the first crystal structure of a CCG-repeat sequence, which shows that two dT(CCG)3 A strands can associate to form a tetraplex structure with an i-motif core containing four C:C(+) pairs flanked by two G:G homopurine base pairs as a structural motif. The tetraplex core is attached to a short parallel-stranded duplex. Each hairpin itself contains a central CCG loop in which the nucleotides are flipped out and stabilized by stacking interactions. The helical twists between adjacent cytosine residues of this structure in the i-motif core have an average value of 30°, which is greater than those previously reported for i-motif structures.
authors.affiliation: Institute of Genomics and Bioinformatics and Institute of Biochemistry, National Chung Hsing University, No. 250 Kuo-Kuang Road, Taichung (Taiwan).
authors.forename: Yi-Wen
authors.initials: YW
authors.lastname: Chen
authors.forename: Cyong-Ru
authors.initials: CR
authors.lastname: Jhan
authors.forename: Stephen
authors.initials: S
authors.lastname: Neidle
authors.forename: Ming-Hon
authors.initials: MH
authors.lastname: Hou
mesh_terms: D018360:Crystallography, X-Ray; D004247:DNA / Q000737:chemistry*; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D018911:Trinucleotide Repeats
chemical_list: D004247:DNA
keywords: DNA structures; X-ray diffraction; i-motifs; neurological disease; tetraplex structures