The RAG1 homeodomain recruits HMG1 and HMG2 to facilitate recombination signal sequence binding and to enhance the intrinsic DNA-bending activity of RAG1-RAG2

V(D)J recombination is initiated by the specific binding of the RAG1-RAG2 ( RAG1/2) complex to the heptamer-nonamer recombination signal sequences (RSS ). Several steps of the V(D)J recombination reaction can be reconstituted i n vitro with only RAG1/2 plus the high-mobility-group protein HMG1 or HMG2. Here we show that the RAG1 homeodomain directly interacts with both HMG bo xes of HMG1 and HMG2 (HMG1,2). This interaction facilitates the binding of RAG1/2 to the RSS, mainly by promoting high-affinity binding to the nonamer motif. Using circular-permutation assays, we found that the RAG1/2 complex bends the RSS DNA between the heptamer and nonamer motifs. HMG1,2 signific antly enhance the binding and bending of the 23RSS but are not essential fo r the formation of a bent DNA intermediate on the 12RSS. A transient increa se of HMG1,2 concentration in transfected cells increases the production of the final V(D)J recombinants in vivo.