FIGURE 5.

A model of SC, PC and SEC complex formation. The RAG1/2 heterotetramer has a pair of NBDs, which are attached to the bulk of the two RAG1 subunits by a flexible link (motion indicated by a double arrowhead). Once bound to either a 12RSS or a 23RSS to form an SC, the NBD becomes tilted asymmetrically relative to the two active sites (marked by red stars), and the second DNA-binding site can only bind the complementary RSS to form a PC. The PC can also assemble from 12RSS and 23RSS DNAs added simultaneously, whether the DNAs are intact or pre-nicked. DNA cleavage in the PC occurs in the presence of Mg²⁺, leading to SEC formation. HMGB1, which facilitates both SC and PC formation, is omitted in the diagram for clarity.