title: Intercalated cytosine motif and novel adenine clusters in the crystal structure of the Tetrahymena telomere.
abstract: The cytosine-rich strand of the Tetrahymena telomere consists of multiple repeats of sequence d(AACCCC). We have solved the crystal structure of the crystalline repeat sequence at 2.5 A resolution. The adenines form two different and previously unknown clusters (A clusters) in orthogonal directions with their counterparts from other strands, each containing a total of eight adenines. The clusters appear to be stable aggregates held together by base stacking and three different base-pairing modes. Two different types of cytosine tetraplexes are found in the crystal. Each four-stranded complex is composed of two intercalated parallel-stranded duplexes pointing in opposite directions, with hemiprotonated cytosine-cytosine (C.C+) base pairs. The outermost C.C+base pairs are from the 5'-end of each strand in one cytosine tetraplex and from the 3'-end of each strand in the other. The A clusters and the cytosine tetraplexes form two alternating stacking patterns, creating continuous base stacking in two perpendicular directions along the x - and z -axes. The adenine clusters could be organizational motifs for macromolecular RNA.
authors.affiliation: Department of Biology, Room 68-233, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
authors.forename: L
authors.initials: L
authors.lastname: Cai
authors.forename: L
authors.initials: L
authors.lastname: Chen
authors.forename: S
authors.initials: S
authors.lastname: Raghavan
authors.forename: R
authors.initials: R
authors.lastname: Ratliff
authors.forename: R
authors.initials: R
authors.lastname: Moyzis
authors.forename: A
authors.initials: A
authors.lastname: Rich
mesh_terms: D000225:Adenine / Q000737:chemistry*; D000818:Animals; D020029:Base Pairing; D003460:Crystallization; D018360:Crystallography, X-Ray; D003596:Cytosine / Q000737:chemistry*; D016054:DNA, Protozoan / Q000737:chemistry*; D004583:Electrons; D008958:Models, Molecular; D009690:Nucleic Acid Conformation; D016615:Telomere / Q000737:chemistry*; D013768:Tetrahymena / Q000737:chemistry* / Q000235:genetics
chemical_list: D016054:DNA, Protozoan; D003596:Cytosine; D000225:Adenine