... THE TYRANNY OF THE LATTICE ...

A systematic comparison of crystal structures of nine different B-DNA dodecamers, in three different space groups, with and without A-tracts , shows that crystal packing or lattice forces are of secondary import ance for helix axis bending, minor-groove width, and propeller twist, While other local helix parameters may be influenced or even establish ed by crystal packing, the properties just enumerated are determined p rimarily by base sequence. One and the same crystal packing scheme can accommodate a bend in one of two different directions, or no bend at all. A-tract regions of B-DNA are inherently straight and unbent, with base-pair inclination no different from that of general-sequence B-DN A. Where bends are observed at junctions between G.C and A.T regions, they always involve a roll about base-pair long axes in a direction th at compresses the wide major groove and, hence, are 90 degrees away fr om that necessary for the correctness of the junction model of A-tract bending. The G.C/A.T junction appears to be a flexible hinge, capable of adopting either a straight or a bent conformation under the local influence of weak crystal packing forces. Such forces therefore are a source of information about DNA deformability and not a curse to be de plored. But as an indication of the weakness of crystal packing forces , introduction of a single bromine atom in the major groove is suffici ent to eliminate a bend, although brominated and unbrominated crystals are isomorphous.