ided ▁into ▁four ▁regions . ▁The ▁N - term inal ▁ 2 1 4 ▁am ino ▁ac ids ▁are ▁disp ens able ▁for ▁relax ation ▁of ▁super co iling ▁activity ▁in ▁vit ro ▁and ▁there ▁are ▁four ▁nuclear ▁local ization ▁signals ▁and ▁sites ▁for ▁interaction ▁with ▁other ▁cell ular ▁prote ins ▁within ▁the ▁N - term inal ▁domain . ▁ ▁The ▁N - term inal ▁domain ▁is ▁followed ▁by ▁a ▁highly ▁conser ved , ▁ 4 2 1 ▁am ino ▁acid ▁core ▁domain ▁containing ▁all ▁of ▁the ▁catal yt ic ▁resid ues ▁except ▁the ▁active ▁site ▁ty ros ine . ▁ ▁This ▁is ▁followed ▁by ▁a ▁poor ly ▁conser ved ▁linker ▁domain ▁of ▁ 7 7 ▁am ino ▁ac ids . ▁ ▁Finally ▁there ▁is ▁a ▁ 5 3 ▁am ino ▁acid ▁C - term inal ▁domain . ▁ ▁The ▁active ▁site ▁T yr 7 2 3 ▁is ▁found ▁within ▁the ▁C - term inal ▁domain . ▁ ▁As ▁further ▁summar ized ▁by ▁P omm ier ▁and ▁by ▁Se ol ▁et ▁al ., ▁ ▁T OP 1 ▁breaks ▁the ▁DNA ▁by ▁a ▁trans ester ification ▁reaction ▁using ▁the ▁active ▁site ▁ty ros ine ▁as ▁the ▁nucle oph ile ▁that ▁attacks ▁the ▁DNA ▁ph osph odi ester ▁back bone . ▁ ▁After ▁the ▁T OP 1 ▁c oval ently ▁attach es ▁to ▁the ▁ 3 ' ▁end ▁of ▁the ▁broken ▁str and , ▁super co iling ▁of ▁the ▁DNA ▁is ▁relax ed ▁by ▁controlled ▁rotation ▁of ▁DNA ▁about ▁the ▁int act ▁str and .