Curved DNA: design, synthesis, and circularization.

Curved DNA molecules and unusually small circles have been obtained by ligation of synthetic 21-base precursors: (sequence in text). The ligation resulted in the formation of double-stranded oligo-(precursor)s possessing a strong 10.5-base-pair (bp) periodicity of the runs of adenines. Two-dimensional polyacrylamide gel electrophoresis of the ligation products showed two distinct families of spots: (i) noncircular oligo(precursor)s of 21 to 231 bp (1- to 11-mers) and (ii) four circles from 105 to 168 bp (eluted and analyzed by denaturing gel electrophoresis). The noncircular oligomers exhibited anomalously slow migration, as if they were as much as three times longer than they actually are. The amount of circular products peaked sharply at approximately equal to 126 bp, near which size the circles have been estimated to be nonconstrained both torsionally and in terms of bending. The nonconstrained circularization provides a technique for the direct measurement of the inherent curvature of DNA in solution. From the size of the circles, an estimate of 8.7 degrees is obtained for the absolute value of the AA X TT wedge angle (roll and tilt combined).