Curved helix segments can uniquely orient the topology of supertwisted DNA.

To show that DNA containing sharp sequence-directed curvature can preferentially establish ends of supertwisted domains, a highly curved DNA from Crithidia fasciculata was cloned into two sites separated by 28% in pBR325. When this construct (pJGC2) was examined by electron microscopy, 63% of the supercoiled molecules were branched with three or more arms, and the remaining molecules appeared as linear interwound rods. The distance between the tips of two of the arms for the branched molecules measured within 2% of 28% of the DNA contour for 32% of the pJGC2 molecules, as contrasted with only 6.6% for the poorly branched pBR325 DNA. When one of the curved segments in pJGC2 was replaced by a highly curved fragment from SV40, similar results were obtained.