SUNY at Albany
June 19-23, 2001
The mediation of the A-B DNA helix transition by guanine tracts
The crystal structure of the DNA dodecamer CATGGGCCCATG lies on a structural continuum between the A and B DNA states. The structure shows highly ordered watered structures, such as a double spine of water molecules against each side of the major groove, stabilizing the GC base pairs in an A-like conformation. The different hydration of GC and AT base pairs provides a physical basis for the A-B helix bistability of guanine tracts and the sensitivity of the structural transition to solvent conditions. The predisposition of GpG base steps for the A DNA form is reflected in the distribution of helical parameters in protein-DNA crystal structures in which GpG base steps show the most A-like properties. The crystal structure of CATGGGCCCATG also shows a terminal GC base pair in a highly strained, buckled conformation caused by crystal packing forces. As previous studies have shown crystal packing forces to be weak, this suggests that the energetic differences between varying DNA geometries may be much lower than commonly believed.
Ho-Leung Ng & Richard E. Dickerson
Molecular Biology Institute
Univ. California, Los Angeles
Los Angeles, CA 90095 USA