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Functional kappa light chain genes are formed during B-lymphocyte differentiation by the joining of initially separate V and J gene segments. It has been suggested that the intervening DNA is deleted, however the recent reports of what appear to be the reciprocal products of V and J recombination (back-to-back conserved V and J flanking sequences, called f-fragments) in DNA from mature lymphocytes make a simple deletion model unlikely. An alternative scheme involving unequal sister chromatid exchange has been proposed, supported by the evidence that the f-fragments seem to have segregated from the chromosome carrying the reciprocal complete kappa light chain gene (this and other schemes are briefly reviewed in ref. 8). We report here the analysis of a mouse myeloma (MOPC 41), in which a productive (kappa+) and a non-productive (kappa-) rearrangement has occured, which may help to clarify the mechanism of V-J joining. The aberrant rearrangement has led to the joining of a J1 gene segment to a sequence unrelated to any V gene (L10), and which in the germ line is flanked by a sequence resembling a V region recombination signal sequence. In this case no segregation of the reciprocal recombination products (kappa-41 and f41), which is a required step in sister chromatid exchange models, has taken place. An inversion model provides the simplest explanation of this J rearrangement.

The mechanism of generating immunoglobulin light chain genes by rearrangement ofvariable (V), joining (J), and constant (C) gene segments is still unknown. It has been discussed mostly in terms of excision and deletion of the DNA between the recombined V and J gene segments. However, the finding in DNA digests from the mouse myeloma T of a fragment (called f-T) that contains the 3' flank of a VK and the 5' flank of a JI gene segment argued against a simple deletion mechanism ) Nucleic Acids Res. 8, 1709-1720. The origin of fragment f-T has now been investigated by cloning and determining the sequence of the germ-line V gene segment that apparently participated in its formation. Moreover, analogous fragments containing flanking sequences were isolated from the myelomas MOPC 173 and 41 (f-173 and f-41) and studied by sequence analysis. The ffragments appear to be recombination products ofV-J rearrangements reciprocal to rearranged X genes but, at least in the cases of f-T and f-173, not of the rearranged V genes present in the same tumor cell. This fact is best explained by a sister chromatid exchange mechanism of V-J recombination because, by this model, the rearranged V genes and the reciprocal flank recombination products would segregate into different cells during the following mitosis. The possibility is suggested that there exists in lymphocyte differentiation more than one mechanism of V-J recombination. The genes for the immunoglobulin light chains are generated by recombination of a variable (V) with a joining (J) gene segment that is located near the constant (C) gene region (1-3). In the K light chain system of the mouse, one of many V gene segments is joined to one offour functional J gene segments (2-4). In addition to functional V-J rearrangements, nonfunctional ones have been observed (5-8) that, at least in some cases, result from aberrations at the V-J junction (5-7). The functional and nonfunctional genes have been termed K+ and K-, respectively (4).One of the unsolved problems in the generation of immunoglobulin genes is the mechanism of V-J joining. Hybridization experiments with DNA from a A (1, 3) and a K chain producing tumor (2, 9) indicated that the sequences 3' adjacent to the rearranging germ-line V gene segment and 5' adjacent to the respective J gene segment (3' V and 5' J flanks) are deleted in the V-J recombination process. These results and the identification of complementary hepta-and nonanucleotide boxes in the 3' flanks of V gene segments and the 5' flanks of J gene segments led to the proposal of hybrid stem structures that are excised and deleted (2, 3). Schemes have also been formulated that do not involve pairing of DNA single strands but rather recognition of the original double-stranded regions by a recombining enzyme and subsequent deletion of the DNA between the V and J gene segments (10, 11).One argument against a simple excision-deletion model was the finding, in myeloma T (5, 12) DNA digests, of a fragment in which the 3' flank of a V gene was linked directly...

Two different kappa light chain genes have previously been isolated from one mouse myeloma. The V (variable, abbreviations in ref. 2) gene segments of the two genes were now used to identify their germline counterparts in EcoRI digests of mouse liver DNA. In addition two sets of related V gene segments were found which hybridize with either of the two DNA probes. Five of the V region fragments of one set were cloned in a lambda phage vector and partially characterized by restriction mapping and Southern blot hybridization. Repetitive DNA sequences were found on each of the five fragments as well as on other cloned immunoglobulin gene containing fragments. Cross-hybridization between some but not all of the regions containing repetitive DNA sequences was observed.