DNA topoisomerases such as bacterial topoisomerase I convert supercoiled circular DNA into relaxed DNA.  The DNA Topoisomerase I Assay Kit is based on the principle that the supercoiled DNA and relaxed DNA yield different fluorescent intensity when interact with fluorescence dye H19.  The relaxed DNA suppresses the fluorescent intensity much more than the supercoiled DNA.  Therefore, when the supercoiled DNA is converted into its relaxed form, the fluorescent signal decreases.  The change of fluorescence intensity is used to measure the relaxation reaction.

Each assay kit (Catalog number DRA020K) includes the reaction buffer (T1 Buffer), supercoiled plasmid DNA and fluorescence dye H19 for 20 DNA relaxation reaction assays in a 96-well plate or 40 assays in a 384-well plate.   The reaction buffer is optimized for bacterial topoisomerase I. 

The Reagent Set (Catalog No.  DRA1000) is designed for drug discovery HTS uses. Each reagent set includes the reaction buffer and fluorescence dye H19 for 1000 assays of DNA relaxation reactions in a 96-well plate assay format or 2000 assays in a 384-well assay format.   Supercoiled DNA is not included in the set.  Supercoiled plasmid pBR322 from any commercial source can be used.