Evidence from base-pair kinetics for two types of adenine tract structures in solution: their relation to DNA curvature.

We have measured the base-pair lifetimes in oligodeoxynucleotides containing tracts of A.T base pairs using imino proton magnetic resonance. When the tract contains more than four consecutive A.T base pairs, possibly including a 5'-AT step but not a 5'-TA step, anomalously long lifetimes are observed. For example, the lifetimes of the central A.T base pairs of the dodecamer 5'-d-CGCAAAAAAGCG are 122 and 91 ms at 15 degrees C whereas, in the same conditions, the lifetime of the central A.T pair of the decamer 5'-d-CGCGATCGCG is only 4 ms, a value similar to those measured in several other B-DNA oligoduplexes [Leroy et al. (1988) J. Mol. Biol. 200, 223-238]. This strongly suggests that, in tracts of four A.T pairs or more, a conformation distinct from standard B-DNA is formed cooperatively. All sequences known to generate curved DNA exhibit anomalously long base-pair lifetimes. This is the first local and physical property shown to correlate with DNA curvature. Our observations suggest that the structure responsible for the long lifetimes is involved in the curvature of DNA.