Structure of a Parallel-Stranded Tetramer of the Oxytricha Telomeric DNA Sequence dT₄G₄

We report here the determination of the solution structure of the single-copy tetramer of the sequence dT₄G₄, the unit repeat in telomeres of the ciliated protozoan Oxytricha, in the presence of potassium ions. This is accomplished by a combination of sequential assignments and distance determinations from 2D proton NMR with model building, based on conformational analysis of the structure using a full-matrix NOESY simulation and molecular dynamics. Each strand in this tetramer structure has an identical environment and conformation: a parallel-stranded, right-handed helix, with all nucleotides in the C2′-endo, anti configuration. The T flanking the G cluster stacks in a 4-fold symmetrical helical array, while the remaining T's become increasingly flexible and sample multiple stacked configurations.