The mechanism of type IA topoisomerase-mediated DNA topological transformations.

Type IA DNA topoisomerases possess several domains forming a toroidal molecule with a central hole large enough to accommodate single- or double-stranded DNA. The sign inversion model predicts several protein–DNA intermediates, including those in which DNA is trapped within the hole. Opposing cysteine residues were incorporated into two independent domains surrounding the putative DNA binding cavity of E. coli topoisomerase III, creating a molecule that can be covalently closed or opened by oxidizing or reducing the disulfide bond. The formation of the disulfide bond allowed the trapping of single- and double-stranded DNA within the cavity of the enzyme and the identification of other intermediates proposed by the sign inversion model.