Bent DNA: visualization of a base-paired and stacked A-B conformational junction.

A model for the junction of contiguous DNA segments having A-DNA and B-DNA conformations is generated using a computerized linked-atom, least-squares model building program. The junction region comprises one base pair and the two neighboring internucleotide linkages and exhibits full hydrogen-bonded base-pairing, full base-stacking, and unexceptional stereochemistry. In addition, the junction has a mixed sugar ring pucker with the junction base pair adopting C2-endo and C3-endo furanose sugar rings in the complementary strands. Since the junction is fully base-stacked, the differences in base tilt between A-DNA and B-DNA result in a bend of 26 degrees in the duplex at the junction. The results of this study indicate: 1) a correlation of the B leads to A transition with several features of the initiation of RNA transcription, 2) possible structural roles of alternating AT and GC sequences in protein recognition, and 3) the possibility of dynamic conformational discontinuities in a DNA helix.