Effects of Mg2+ on DNA cleavage mediated by wild-type and GrlAS81F topoisomerase IV in the presence of ciprofloxacin and 8-methyl-quinazoline-2,4-dione. Results are shown for 50 μM ciprofloxacin (Cipro, left panel) and 10 μM 8-methyl-quinazoline-2,4-dione (Dione, right panel) with the wild-type (WT, black squares) and GrlAS81F (S81F, red circles) enzymes. DNA cleavage for each drug-enzyme pair was normalized to 100% at 10 mM Mg2+ to facilitate direct comparisons. Error bars represent the standard deviation of three or more independent experiments.