The entire nucleotide sequence of a 1.7-kilobase embryonic DNA fragment containing five joining (J) DNA segments for mouse immunoglobulin kappa chain gene has been determined. Each J DNA segment can encode amino acid residues 96--108. Comparison of one of the five J DNA sequences with those of an embryonic variable (V) gene and a complete kappa chain gene permitted localisation of a precise recombination site. The 5'-flanking regions of J DNA segments could form an inverted stem structure with the 3'-non-coding region of embryonic V genes. This hypothetical structure and gel-blotting analysis of total embryo and myeloma DNA suggest that the somatic recombination may be accompanied by excision of an entire DNA segment between a V gene and a J DNA segment. Antibody diversity may in part be generated by modulation of the precise recombination sites.

We have examined the amino-terminal sequence of the Kc light chains of a set of monoclonal antibodies specific for one of the major antigenic determinants (Sb) on the influenza virus PR8[A/PR/8/34(HlNl)J hemagglutinin molecule. This set was believed to be structurally related from earlier serological analysis that typed these K chains as members of the variable (V) region VK21 group [Staudt, L. M. & Gerhard, W. (1983) J. Exp. Med. 157,. Our sequence analysis confirms and extends this conclusion; all examples of this set belong to a subgroup of the VK21 group, V,,21C. A special feature of this set of K light chains is that all examples were derived from the same mouse (designated H36). This sequence analysis along with the characterization of gene rearrangements at the K light chain loci of these hybridomas is consistent with the idea that certain members of this set are the progeny of one or two lymphocytes. Because of this potential clonal relationship, we can reach several conclusions about the diversity observed among these K light chains: (') the diversity is due to somatic mutation, (it) somatic mutations occur sequentially and accumulate in the first complementarity-determining region, and (iii) the extent of somatic variation in this sample is high, suggesting a somatic mutation rate of about 10-3 per base pair per generation.Antibody diversity arises from several sources. Individuals inherit multiple variable (V) region gene segments for both heavy (VH) and light (VK, VA) chains, joining (J) gene segments (JH, J,, Jh), and diversity (D) gene segments (DH).The initial antibody repertoire of an individual is a product of the combinatorial joining of these gene segments, i.e., V4s with JKS or different VH, DH, and JH combinations, that form complete V1 or VH genes. Errors committed during the process of joining contribute additional diversity to this repertoire (reviewed in ref. 1). Finally, that somatic mutation further amplifies this germ-line repertoire seems to be established (2). The original evidence for somatic mutation favored a model by which point mutations accumulate sequentially during cell division (3). Other models link somatic mutation with specific events during lymphocyte differentiation (4, 5) and propose cataclysmic mechanisms of mutagenesis that introduce multiple amino acid substitutions in one step (6, 7). These models are based on the comparison of V region sequences of independently induced plasmacytoma and hybridoma antibodies to their putative germ-line counterparts. Hence, little can be concluded about the time course of somatic mutation.A better understanding of the nature of somatic mutation can be reached by comparisons of the V genes of a cell lineage. Scharff and colleague have analyzed certain mutants and revertants of the cell line S107 and conclude that the in vitro rate of mutation at the VH gene expressed in this plasmacytoma is significantly higher than that of nonimmunoglobulin genes (8). A possible in vivo analogy is described here: we have initiated a structu...

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