▁bond ▁cle av age . ▁ ▁However , ▁it ▁is ▁believed ▁that ▁H ind III ▁util izes ▁a ▁common ▁mechanism ▁of ▁recognition ▁and ▁catal ysis ▁of ▁DNA ▁found ▁in ▁other ▁type ▁II ▁en zym es ▁such ▁as ▁E co RI , ▁B am HI , ▁and ▁B gl II . ▁ ▁These ▁en zym es ▁contain ▁the ▁am ino ▁acid ▁sequence ▁mot if ▁P D -( D / E ) X K ▁to ▁coordinate ▁M g 2 + , ▁a ▁c ation ▁required ▁to ▁cle ave ▁DNA ▁in ▁most ▁type ▁II ▁restriction ▁end on uc leases . ▁ ▁The ▁c of actor ▁M g 2 + ▁is ▁believed ▁to ▁bind ▁water ▁mole cules ▁and ▁carry ▁them ▁to ▁the ▁catal yt ic ▁sites ▁of ▁the ▁en zym es , ▁among ▁other ▁c ations . ▁ ▁Un like ▁most ▁documented ▁type ▁II ▁restriction ▁end on uc leases , ▁H ind III ▁is ▁unique ▁in ▁that ▁it ▁has ▁little ▁to ▁no ▁catal yt ic ▁activity ▁when ▁M g 2 + ▁is ▁subst it uted ▁for ▁other ▁c of act ors , ▁such ▁as ▁M n 2 + . ▁ ▁Site - direct ed ▁mut agen esis ▁ ▁Despite ▁the ▁uncertainty ▁concerning ▁the ▁structure - catal ysis ▁relationship ▁of ▁type ▁II ▁end on uc leases , ▁site - direct ed ▁mut agen esis ▁of ▁the ▁restriction ▁end on uc lease ▁H ind III ▁has ▁provided ▁much ▁insight ▁into ▁the ▁key ▁am ino ▁acid ▁resid ues ▁involved . ▁ ▁In ▁particular , ▁substitution s ▁of ▁As n ▁for