Catalytic function of topo I in cultured SSc fibroblasts with and without SUMO1 siRNA transfection. A serial dilution of the nuclear extract containing topo I obtained from SSc fibroblasts was used to relax 0.25 μg supercoiled DNA. In this figure, the supercoiled DNA band was completely transformed to relaxed DNA at dilutions of one half and one in the fibroblasts without siRNA transfection or non-target siRNA transfection. In contrast, this change was observed between the one-eighth and one-fourth dilutions in the fibroblasts with SUMO1 transfection, which indicates a higher efficiency of catalytic function of topo I after SUMO1 inhibition in the fibroblasts. According to the intensity of the bands of remaining supercoiled DNA in serial dilutions in the assays of three fibroblast strains, these changes are significant. The P-values are 0.045 and 0.027 at the one-fourth dilution for comparisons between SUMO1 siRNA vs. non-target siRNA, or vs. without siRNA transfected fibroblasts, respectively (Student's t-test). This is representative of three SSc fibroblast strains examined in SUMO1 siRNA studies. *A, supercoiled DNA; B, relaxed DNA.