Renaturation of complementary single-stranded DNA circles: complete rewinding facilitated by the DNA untwisting enzyme.

Renaturation of two complementary single-stranded circles should be limited by topological constraints against the rewinding of the DNA helix. If a mixture of complementary single-stranded rings is annealed and then treated with the DNA untwisting enzyme, the DNA circles completely renature as judged by (i) the presence of interlocked rings that sediment at 53 S in alkali, (ii) the buoyant density of the renatured DNA in CsCl gradients containing ethidium bromide, and (iii) the resistance of the product to the single-strand-specific S1 nuclease. Therefore, the DNA untwisting enzyme is able to provide a transient single-strand break that is sufficient to allow the two strands to completely rewind. The possibility that the untwisting enzyme might facilitate the initiation of the process of genetic recombination is discussed.