Two are better than one

A crystal structure of a tetrameric restriction enzyme, NgoMIV, bound to tw o DNA duplexes has been determined. Two subunits contact each duplex in muc h the same manner as a dimeric restriction enzyme recognizing a single site , but the dimeric units are packed back-to-back, placing the duplexes on op posite sides of the tetramer. Interaction with two recognition sites, presu mably via looping, enhances NgoMIV activity.