JUNCTIONAL DIVERSITY IN SIGNAL JOINTS FROM T-CELL RECEPTOR-BETA AND RECEPTOR-DELTA LOCI VIA TERMINAL DEOXYNUCLEOTIDYL TRANSFERASE AND EXONUCLEOLYTIC ACTIVITY

The site-specific V(D)J recombination reaction necessary to assemble t he genes coding for immunoglobulin (Ig) and T cell receptor (TCR) vari able regions is initiated by a precise double strand cut at the border of the recombination signals nanking the genes. Extensive processing of the coding ends before their Ligation accounts for most of the Ig a nd TCR repertoire diversity. This processing includes both base additi ons to and loss from the coding ends. On the other hand, it has genera lly been thought that signal ends are not modified before they an fuse d, and that signal joints consist of a perfect head-to-head ligation o f the recombination signals. In this study, we analyzed signal joints created during the rearrangement of different TCR-beta and TCR-delta g enes in thymocytes. We show that a significant fraction (up to 24%) of these signal joints exhibits junctional diversity. This diversity res ults from N nucleotide additions for TCR-beta signal joints, and from N additions and exonucleolytic digestion for TCR-delta joints. Altoget her, our findings suggest that: (a) signal ends can undergo some of th e same modifications as coding ends, (b) inversional rearrangement gen erates more diversity than deletional events, and (c) fine differences exist in the recombinase/DNA complexes formed at each rearranging loc us.