CRYSTALLOGRAPHIC ANALYSIS OF C-C-A-A-G-C-T-T-G-G AND ITS IMPLICATIONSFOR BENDING IN B-DNA

Stacked B-DNA double helices of sequence C-C-A-A-G-C-T-T-G-G exhibit t he same 23-degrees bend at -T-G-G C-C-A- across the nonbonded junction between helices that is observed in the middle of the decamer helix o f sequence C-A-T-G-G-C-C-A-T-G, even though the space group (hexagonal vs orthorhombic), crystal packing, and connectedness at the center of the bent segment are quite different. An identical bend occurs across the interhelix junction of every monoclinic crystal structure of sequ ence C-C-A-x-x-x-x-T-G-G, suggesting that T-G-G-C-C-A constitutes a na tural bending element in B-DNA. The bend occurs by rolling stacked bas e pairs about their long axes; there is no ''tilt'' component. Of the three possible models for A-tract bending-bent-A-tract,junction bends, or bent-non-A-which cannot be distinguished by solution measurements, all crystallographic evidence over the past 10 years unanimously supp orts the non-A regions as the actual bending loci.