Occurrence, solution structure and stability of DNA hairpins stabilized by a GA/CG helix unit.

The occurrence and NMR solution structure of a class of biloop hairpins containing the sequence 5'-CGXYAG are presented. These hairpins, which are variations on a sequence found in the reverse transcript of the human T-cell leukemia virus 2 (HLV2), show elevated melting points and high chemical stability toward denaturation by urea. Hairpins with the 5'-CGXYAG configuration have melting points 18-20 degrees higher than hairpins with 5'-CAXYGG or 5'-GGXYAC configurations. The identities of the looping bases, X and Y above, play a negligible role in determining the stability of this DNA hairpin stability. This is very different from G-A based loops in RNA, where the third base must be a purine for high stability [the GNRA loops; V.P. Antao, S.Y. Lai and I. Tinoco, Jr (1991) Nucleic Acids Res., 19, 5901-5905]. We show that these properties are associated with a four base helix unit that contains both a sheared GA base pair and a Watson-Crick CG base pair upon which it is stacked. As an understanding of the significance of AG base pairs has become increasingly important in the structural biology of nucleic acids, we compute an 0.7-0.9 A precision ensemble of NMR solution structures using iterative relaxation matrix methods. Calculations performed on NMR-derived structures indicate that neither base-base electrostatic interactions, nor base-solvent dispersive interactions, are significant factors in determining the observed differences in hairpin stability. Thus the stability of the 5'-CGXYAG configuration would appear to derive from favorable base-base London/van der Waals interactions.