MOST COMPACT HAIRPIN-TURN STRUCTURE EXERTED BY A SHORT DNA FRAGMENT, D(GCGAAGC) IN SOLUTION - AN EXTRAORDINARILY STABLE STRUCTURE RESISTANT TO NUCLEASES AND HEAT

The three-dimensional structure of a short DNA fragment, d(GCGAAGC) exhibiting an extraordinarily stable hairpin structure was determined by nuclear magnetic resonance spectroscopy. Two possible models were obtained by molecular modelling using distance and torsion constraints. Only one of these two models is the correct structure, which can clearly explain all the H-1 chemical shifts. d(GCGAAGC) is folded back on itself between A(4) and A(5) and all the sugars in the fragment adopt the C2'-endo conformation. This compact molecule is stabilized by regular extensive base-stacking interaction within each B-form helical strand of G(1)C(2)G(3)A(4) and A(5)G(6)C(7), and by two G-C and one G(3)-A(5) base pairs. The molecule is hard to differentiate into stem and loop regions, so that we classify it as a turn (hairpin-turn) structure exerted by a single-stranded DNA. This highly stacked structure shows high thermostability and strong resistance against nucleases contained in E. coli extracts and in human serum.