Using a substrate measuring deletion or inversion of an I-SceI-excised fragment and both accurate and inaccurate rejoining, we determined the impact of non-homologous end-joining (NHEJ) on mammalian chromosome rearrangements. Deletion is 2- to 8-fold more efficient than inversion, independent of the DNA ends structure. KU80 controls accurate rejoining, whereas in absence of KU mutagenic rejoining, particularly microhomology-mediated repair, occurs efficiently. In cells bearing both the NHEJ and a homologous recombination (HR) substrate containing a third I-SceI site, we show that NHEJ is at least 3.3-fold more efficient than HR, and translocation of the I-SceI fragment from the NHEJ substrate locus into the HR-I-SceI site can occur, but 50- to 100-fold less frequently than deletion. Deletions and translocations show both accurate and inaccurate rejoining, suggesting that they correspond to a mix of KU-dependent and KU-independent processes. Thus these processes should represent prominent pathways for DSB-induced genetic instability in mammalian cells.

We report the first and complete nucleotide sequence of a human gamma 3 heavy chain constant region gene (Cy3). This gene displays the same organization than the others Cy genes and exhibits normal RNA splice and polyadenylation sites. A comparison of its primary sequence with those of Cy 1, Cy2 and Cy4 genes confirms the high degree of homology (95%) of the human family in both coding and non-coding regions, and the divergence of the hinge region. The Cy3 gene we sequenced codes for a Gm(b) y3 chain (EZZ).Comparison with other known protein sequences reveals that only two specific aminoacids are involved in the Gm(b) and Gm(g) allotypes, which suggests an important part of the spatial configuration in the allotypic specificities.

Two new V gamma genes in humans are described from rearrangement in T cell lines, which constitute single members of new V gene subgroups of the T‐cell rearranging gamma (TRG gamma) locus. These two genes (herein designated as belonging to V gamma III and V gamma IV subgroups) are located between V gamma I/V gamma II subgroups and the constant (C) gamma genes. The existence of these new genes brings the number of different, potentially useable, human TRG V gamma genes to eight (excluding at least five pseudo V gamma genes) and the number of distinct subgroups to four. Polymorphism in the sequence of the V gamma II subgroup gene is also described and rearranged fragment sizes which make possible an unequivocal assignment of a V gamma rearrangement are given. These results extend previous conclusions of the inherited diversity of the human TRG V gamma locus.