Hairpin and parallel quartet structures for telomeric sequences

Abstract

The role of thymine residues in the formation of G-quartet structures for telomeric sequences has been investigated using model oligonucleotides of the type d(G₄T_nG₄4), with n = 1-4. Sequences d(G₄T₃G₄) and d(G₄T₄G₄) adopt a G-quartet structure formed by hairpin dimerization in 70 mM NaCI as judged by a characteristic circular dichrolsm signature with a 295 nm positive and 265 nm negative bands while d(G₄T G₄) adopts a parallel G-quartet structure like d(G12) which exhibits a strong positive band at 260 nm and a negative band at 240 nm. The sequence d(G₄T₂G₄) exhibits a mixture of both conformations. The stability of hairpin G-quartet structures decreases with decrease in the number of Intervening thymine residues. Potassium permanganate, a single strand specific probe has been used to establish the presence of loops composed of T residues in the hairpin G quartet structures formed by the oligonucleotides d(G₄T_nG₄) with n = 2-4 in 70 mM NaCI. The formation of hairpin G quartet structure for the above sequences is further supported by the enhanced electrophoretic mobility observed on non-denaturing polyacrylamide gels. Human telomeric sequence d(TTAGGG)₄ which showed enhanced electrophoretic mobility like Tetrahymena telomeric sequence d(T₂G₄)₄ also exhibited a characteristic CD spectrum for a folded-back G-quartet structure. A detailed model for G-quartet structure involving hairpin dimer with alternating synanti-synanti conformation for the guanine residues both along the chain as well as around the G tetrad with at least two thymine residues in the loop is proposed. Intermolecular association of short telomeric sequences reported here provides a possible model for chromosomal pairing.