V(D)j Recombination Becomes Accessible

The diverse antigen receptor repertoires of B and T lym-phocytes are generated by somatic rearrangement of Ig and TCR V, D, and J gene segments during lymphocyte development. A basic mechanistic understanding of V(D)J re-combination reaction is now at hand. The process is initiated by recombinase proteins recombination activating gene (RAG)-1 and RAG-2, which bind to recombination signal sequences (RSSs) that flank Ig and TCR gene segments, recruit a pair of RSSs into a synaptic complex, and generate double strand breaks (DSBs) between the RSSs and coding segments. The reaction is then completed with the additional participation of DSB repair proteins, which help to process and rejoin the coding and signal end molecules to generate coding joints and signal joints, respectively. V(D)J recombination at the various Ig and TCR loci is highly regulated during lymphocyte development, with lineage-and developmental stage–specific rearrangement events distinguishing individual loci, and even individual gene segments within loci. It has long been appreciated that this developmental regulation cannot be accounted for by the expression of either RAG or DNA repair proteins. It has been noted that transcription of unrearranged gene segments (germline transcription) parallels their developmental activation for V(D)J recombination. Yancopoulos and Alt interpreted this transcriptional activity to reflect a permissive chromatin structure, and on this basis initially proposed that a permissive chromatin structure determines the suit-ability of particular gene segments as targets for the V(D)J recombinase (1). Thus, developmental control of V(D)J re-combination would be exerted at the level of gene segment accessibility within chromatin. Accessibility control has been a tremendously useful concept that has influenced research in this area for quite some time (2). However, gaining a molecular understanding of accessibility has been difficult. The accessibility hypothesis received perhaps its strongest confirmation when Stanhope-Baker and Schlissel showed that chromosomal RSSs could be cleaved by introducing exogenous RAG proteins into isolated nuclei in vitro (3). The ability of particular RSSs to serve as substrates for RAG was determined by the developmental stage of the cells from which the nu-clei were isolated, an indication of developmentally regulated changes in RSS accessibility. In addition, a host of studies have established that transcriptional enhancers and promoters play critical roles in establishing the efficiency, lineage specificity, and developmental stage specificity of V(D)J recombination events (2). However, the molecular mechanisms by which promoter and enhancer function translate to accessibility for V(D)J recombination have remained elusive. Chromatin exists in a highly …