Structure and stability of human telomeric sequence.

Telomeric DNA of a variety of vertebrates including humans contains the tandem repeat d(TTAGGG)n. We have investigated the structural properties of the human telomeric repeat oligonucleotide models d(T2AG3)4, d(G3T2A)3G3, and d(G3T2AG3) using CD, gel electrophoresis, and chemical probing techniques. The sequences d(G3T2A)3G3 and d(T2AG3)4 assume an antiparallel G quartet structure by intramolecular folding, while the sequence d(G3T2AG3) also adopts an antiparallel G quartet structure but by dimerization of hairpins. In all the above cases, adenines are in the loop. The TTA loops are oriented at the same end of the G tetrad stem in the case of hairpin dimer. Further, the oligonucleotide d(G3T2AG3) forms a higher order structure by the association of two hairpin dimers via stacking of G tetrad planes. Here we show that N-7 of adenine in the hairpin dimer is Hoogsteen hydrogen-bonded. The partial reactivity of loop adenines with DEPC in d(T2AG3)4 suggests that the intramolecular G quartet structure is highly polymorphic and structures with different loop orientations and topologies are formed in solution. Intra- and interloop hydrogen bonding schemes for the TTA loops are proposed to account for the observed diethyl pyrocarbonate reactivities of adenines. Sodium-induced G quartet structures differ from their potassium-induced counterparts not only in stability but also in loop conformation and interactions. Thus, the overall structure and stability of telomeric sequences are modulated by the cation present, loop sequence, and the number of G tracts, which might be important for the telomere function.