Closed circular DNA as a probe for protein-induced structural changes.

Biological systems are replete with examples of DNA formed into a closed loop structure, either alone or in close association with proteins. Such closed circular DNA molecules are subject to a topological constraint that modifies, often in a major way, the structure and reactivity of the DNA. The topological constraint also permits closed circular DNAs to be used as analytical tools to learn about the structure of DNA-protein complexes.