Analysis of local helix bending in crystal structures of DNA oligonucleotides and DNA-protein complexes.

Sequence-dependent bending of the helical axes in 112 oligonucleotide duplex crystal structures resident in the Nucleic Acid Database have been analyzed and compared with the use of bending dials, a computer graphics tool. Our analysis includes structures of both A and B forms of DNA and considers both uncomplexed forms of the double helix as well as those bound to drugs and proteins. The patterns in bending preferences in the crystal structures are analyzed by base pair steps, and emerging trends are noted. Analysis of the 66 B-form structures in the Nucleic Acid Database indicates that uniform trends within all pyrimidine-purine and purine-pyrimidine steps are not necessarily observed but are found particularly at CG and GC steps of dodecamers. The results support the idea that AA steps are relatively straight and that larger roll bends occur at or near the junctions of these A-tracts with their flanking sequences. The data on 16 available crystal structures of protein-DNA complexes indicate that the majority of the DNA bends induced via protein binding are sharp localized kinks. The analysis of the 30 available A-form DNA structures indicates that these structures are also bent and show a definitive preference for bending into the deep major groove over the shallow minor groove.