B-DNA TWISTING CORRELATES WITH BASE-PAIR MORPHOLOGY

The observed sequence dependence of the mean twist angles in 38 B-DNA crystal structures can be understood in terms of simple geometrical fe atures of the constituent base-pairs. Structures with low twist appear to unwind in response to severe steric clashes of large exocyclic gro ups (such as NH2-NH2) in the major and minor grooves, while those with high twist are subjected to lesser contacts (H-O and H-H). We offer a simple clash function that depends on base-pair morphology (i.e. the chemical constitution of base-pairs) and satisfactorily accounts for t he twist angles of the ten common Watson-Crick dimer steps both in the solid state and in solution. The twist-clash correlation that we find here still holds when extended to modified bases. In addition to Call adine's purine-purine clashes, we add other close contacts between bas es in the grooves, and consider the conformational restrictions on the geometry of the sugar-phosphate backbone (namely, we emphasize the te ndency of DNA to conserve virtual backbone length). The significance o f this finding is threefold: (1) sequence-dependent DNA twisting is di rectly involved in protein-DNA interactions; (2) strong correlation be tween Twist and Roll helps to elucidate the bending of the double heli x as a function of base sequence; (3) it is possible to anticipate the effects of chemical modifications on twisting and bending. The mutual correlations of other structural parameters with the twist make this angle a primary determinant of DNA conformational heterogeneity.