202 Immunoglobulin Heavy Chain Diversity Dna Segments

Complete, active, immunoglobulin variable (V) 1 genes are generated by somatic rearrangement of DNA sequences during the differentiation of lymphocyte precursor cells (1, 2). In the mouse, the conventionally defined V region of the light chains, both and ~¢ types, consists of two DNA segments: VL and Je, which are separate in the germline genome. The two DNA segments are joined at the 3' end of the V DNA and the 5' end of the J DNA to create a complete, light chain V gene (3-8). In contrast, the heavy chain variable gene is encoded by three DNA segments, Vn, diversity (D) (which encodes primarily the third hypervariable region) (9), and JH. Both VH-D and D-JH joinings are necessary to generate a complete, heavy chain V gene (10, 11). Sequencing studies revealed the presence of two blocks of highly conserved sequences around the recombination sites. In the 5' flanking region of the J DNAs, a palindromic heptamer, CACTGTG, and a T-rich nonamer, G G T T T T T G T , are conserved, and the sequences complementary to the heptamer and nonamer are ubiquitous in the 3' flanking region of the germline V DNA segments. It has been proposed that these two blocks of sequences constitute a recognition signal for the putative recombinase (7, 8). The lengths of the spacers between the heptamer and the nonamer are strikingly regular (10, 11). The V~ and J~ DNA segments have spacers of 23 and 12 base pairs (bp), respectively (3, 12). In contrast, the Vx and Jg spacers are 12 ± 1 and 23 ± 1 bp, respectively (7, 8). In addition, the spacers of all VH and JHare 23 ± 1 bp (10, 11, 13-15). We and others proposed that the putative recombinase for V-J and V-D-J joinings contains two DNA-binding proteins: one recognizing the signal sequences with a shorter (12 bp) spacer and the other with a longer (23 bp) spacer (12/23-bp spacer rule), and all recombinations leading to the generation of a complete V gene, namely, Vt¢ and J~, VJk and J~, VH and D, and D and JH joinings, are mediated by the same or similar enzymes following this rule (10, 11). This proposal predicted that the germline D segment would carry the signal sequences with a shorter spacer on each side. This prediction was met by our recent study: two DNA segments identified on the germline genome carried the conserved sequences with a 12-bp spacer on each side (16, 17). We now describe the structure and organization of more germline D segments and present a model explaining how these D segments might become part of the complete VH gene active in myelomas during differentiation of lymphocytes. * Present address is the Massachusetts Institute of Technology, Center for Cancer Research, room E 17350, 77 Massachusetts Ave., Cambridge, Massachusetts 02139. l Abbreviations used in this paper." D, diversity; kb, kilobase; V, variable; bp, base pairs.