Localized epigenetic changes induced by DH recombination restricts recombinase to DJH junctions

Genes encoding immunoglobulin heavy chains (Igh) are assembled by rearrangement of variable (V-H), diversity (D-H) and joining (J(H)) gene segments. Three critical constraints govern V-H recombination. These include timing (V-H recombination follows D-H recombination), precision (V-H gene segments recombine only to DJ(H) junctions) and allele specificity (V-H recombination is restricted to DJ(H)-recombined alleles). Here we provide a model for these universal features of V-H recombination. Analyses of DJ(H)-recombined alleles showed that DJ(H) junctions were selectively epigenetically marked, became nuclease sensitive and bound RAG recombinase proteins, which thereby permitted D-H-associated recombination signal sequences to initiate the second step of Igh gene assembly. We propose that V-H recombination is precise, because these changes did not extend to germline D-H segments located 5' of the DJ(H) junction.