Joining-deficient RAG1 mutants block V(D)J recombination in vivo and hairpin opening in vitro

The RAG proteins cleave at V(D)J recombination signal sequences then form a postcleavage complex with the broken ends. The role of this complex in end processing and joining, if any, is undefined. We have identified two RAG1 mutants proficient for DNA cleavage but severely defective for coding and s ignal joint formation, providing direct evidence that RAG1 is critical for joining in vivo and strongly suggesting that the postcleavage complex is im portant in end joining. We have also identified a RAG1 mutant that is sever ely defective for both hairpin opening in vitro and coding joint formation in vivo. These data suggest that the hairpin opening activity of the RAG pr oteins plays an important physiological role in V(D)J recombination.