Dynamic and Energetic Signatures of Adenine Tracts in a rA-dT RNA-DNA Hybrid and in Homologous RNA-DNA, RNA -RNA, and DNA-DNA Double Helices

Nuclear magnetic resonance spectroscopy and proton exchange are being used to characterize the opening reactions of individual base pairs in the RNA-DNA hybrid 5'-rGCGAUAAAAAG-GCC-3'/5'-dGGCCTTTTTATCGC-3'. The hybrid contains a central tract of five rA-dT base pairs. The rates and the equilibrium constant of the opening reaction for each base pair are determined from the dependence of the exchange rates of imino protons on ammonia concentration, at 10 degrees C. The results are compared to those previously obtained by our laboratory for three homologous duplexes of the same base sequence (except for the appropriate T/U substitution), containing tracts of dA-rU, rA-rU, or dA-dT base pairs. The rA-dT tract is distinguished by an enhanced propensity of the base pairs to exist in the extrahelical state. The opening rates of rA-dT base pairs also exhibit a strong dependence on the location of the base pair in the structure; namely, as one advances into the tract, the opening rates of rA-dT base pairs gradually decrease. The local stability of each rA-dT base pair within the tract is the same as that of the corresponding rA-rU base pair in the homologous RNA-only duplex but differs from the stabilities of dA-dT and dA-rU base pairs in the other two duplexes (namely, dA-dT > rA-dT > dA-rU). These results demonstrate that, in nucleic acid double helices with the same base sequence, the opening dynamics and the energetics of individual base pairs are strongly influenced by the nature of the strand and by the structural context of the base pair.