STRUCTURE OF POLY D(AI)CENTER-DOT-POLY D(CT) IN 2 DIFFERENT PACKING ARRANGEMENTS

X-ray diffraction analysis of poly d(AI).poly d(CT) in oriented and po lycrystalline fibers has revealed the DNA structure to be a 10-fold, r ight-handed, antiparallel, Watson-Crick base paired double helix in tw o distinct packing arrangements corresponding to one and two helices, respectively, in the unit cell. The helix pitch is 32.1 Angstrom and 3 2.4 Angstrom in the two cases, almost 1.5 Angstrom shorter than in cla ssical B-DNA. The resulting B'-DNA geometry, described in terms of a t etranucleotide repeat which is conformationally similar to B-DNA, has its minor groove closely shut and major groove correspondingly widened , thus striking a sharp morphological contrast to B-DNA. According to difference electron density maps, a spine of hydration along the minor groove connects both strands and provides structural stability; order ed sodium ions and water molecules are actively involved in bridging t he phosphate groups of neighboring helices. The crystallographic R-val ues for these two allomorphs are 0.26 and 0.20, respectively, for data up to 3.0 Angstrom resolution.