Two unusual,heterodimeric restriction enzymes have been studied.BsslMI cleaves the sequence 5-GG/GTC-3 and the complementary strand 5-/GACCC-3.BstNBI cleaves the sequence 5-GAGTCNNNN/-3 and leaves a blunt end.Both of these Type liT enzymes are composed of two subunits:a "natural nicking enzyme" which binds the recognition sequence and cuts only one strand,and a smaller subunit which has no activity on its own but,when combined with the larger subunit,cleaves the second strand at the recognition sequence.The two subunits behave differently during column chromatography,and so the restriction enzymes appear to be unstable during purification.The subunits can be purified separately and then mixed together to reconstitute an active,double-strand cleaving restriction enzyme.