DNA topoisomerase IV (the parC-parE complex) have DNA relaxation activity that converts the supercoiled DNA into its relaxed form.   The Topoisomerase IV DNA Relaxation Assay Kit is based on the principle that the relaxed DNA suppresses the fluorescent intensity much more than the supercoiled DNA when the DNA interacts with fluorescence dye H19 (a fluorescence dye for DNA relaxation / supercoiling assay) in the presence of magnesium.  When the supercoiled DNA is converted into its relaxed form, the fluorescent signal decreases. 

E. coli Topo IV DNA Relaxation Assay Kit Plus (Catalog No. T4RA-100KE)

S. pneumoniaeTopo IV DNA Relaxation Assay Kit Plus (Catalog No. T4RA-100KN)

P. aeruginosaTopo IV DNA Relaxation Assay Kit Plus (Catalog No. T4RA-100KP)

S. aureusTopo IV DNA Relaxation Assay Kit Plus (Catalog No. T4RA-100KS)