DNA bending by the bulge defect

Comparative gel electrophoresis measurements were used to characterize DNA bending in molecules containing an extra adenosine on one strand, the so-called bulge defect. We used oligomers containing A6 tracts separated from the bulged base by varying numbers of nucleotides to determine the direction and magnitude of the bulge bend. Helix unwinding by the bulge was determined from the electrophoretic anomaly as a function of the size of the repeated monomers. We conclude that the bulge bend is 21 degrees +/- 3 degrees, primarily in the direction of tilt away from the bulged base. The total helical advance of the DNA at the bulge site is smaller than would be the case if the complementary T were present, corresponding to an unwinding by 25 degrees +/- 6 degrees. These values are in good agreement with the results of NMR and energy minimization studies of the bulged base in double-helical deoxyoligonucleotides [Woodson, S. A., & Crothers, D.M. (1988) Biochemistry 27, 3130-3141]