The critical initial step in V(D)J recombination, binding of RAG1 and RAG2 to recombination signal sequences flanking antigen receptor V, D, and J gene segments, has not previously been characterized in vivo. Here, we demonstrate that RAG protein binding occurs in a highly focal manner to a small region of active chromatin encompassing Ig kappa and Tcr alpha J gene segments and Igh and Tcr beta J and J-proximal D gene segments. Formation of these small RAG-bound regions, which we refer to as recombination centers, occurs in a developmental stage- and lineage-specific manner. Each RAG protein is independently capable of specific binding within recombination centers. While RAG1 binding was detected only at regions containing recombination signal sequences, RAG2 binds at thousands of sites in the genome containing histone 3 trimethylated at lysine 4. We propose that recombination centers coordinate V(D)J recombination by providing discrete sites within which gene segments are captured for recombination.
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