Epigenetic modifications of the V region after DJ recombination in Pro-B cells.

The variable region of murine immunoglobulin heavy chain (Igh) is assembled by sequential D -J and V -DJ recombination. The accessibility of the Igh locus determines the order of rearrangement. Because of the large number of V genes and the lack of a suitable model, the epigenetic modifications of V genes after DJ recombination have not previously been characterized. Here, we employed two v-Abl pro-B cell lines, in which the Igh locus is in germline and DJ -recombined configurations, respectively. The DJ junction displays the characteristics of a recombination centre, such as high levels of activation-associated histone modifications and recombination-activating gene protein (RAG) binding in DJ -rearranged pro-B cells, which extend the recombination centre model proposed for the germline Igh locus. The different domains of the V region have distinct epigenetic characteristics after DJ recombination. Distal V genes have higher levels of active histone modifications, germline transcription and Pax5 binding, and good quality recombination signal sequences. Proximal V genes are relatively close to the DJ recombination centre, which partially compensates for the low levels of the above active epigenetic modifications. DJ recombination centre might serve as a cis-acting element to regulate the accessibility of the V region. Furthermore, we demonstrate that RAG weakly binds to functional V genes, which is the first detailed assessment of RAG dynamic binding to V genes. We provide a way for V -DJ recombination in which the V gene is brought into close proximity with the DJ recombination centre for RAG binding by a Pax5-dependent chromosomal compaction event, and held in this position for subsequent cleavage and V -DJ joining.