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

Abstract

Renaturation of 2 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 the presence of interlocked rings that sediment at 53 S in alkali, the buoyant density of the renatured DNA in CsCl gradients containing ethidium bromide, and the resistance of the product to the single-strand-specific S1 nuclease. The DNA untwisting enzyme can apparently provide a transient single-strand break that is sufficient to allow the 2 strands to completely rewind. The possibility that the untwisting enzyme might facilitate the initiation of the process of genetic recombination is discussed. [SV-40 DNA and rat liver untwisting enzyme are used].