Resistance to a class of antibiotics targeting these enzymes often disrupts a water–metal ion bridge that chelates the drug’s keto acid. One of these enzymes from the vaccinia virus modifies a 3-prime thymidine in a namesake cloning method. An inhibitor of these enzymes was isolated from the barks of Camptotheca trees, derivatives (-5[1])of which include irinotecan and a related drug partially named after them. These enzymes create a “gate” for the T-segment to cross in the “strand passage” mechanism. Ciprofloxacin (“sip-ro-flox-uh-sin”) and other fluoroquinolones (“fluoro-quino-lones”) target (10[1])these enzymes, which can resolve catenanes. Bacteria use a type of these enzymes called gyrase (10[1])that generates negative supercoils and changes the linking number (10[1])by two. For 10 points, name these enzymes that relieve torsional stress in DNA. ■END■ (0[5])