Abstract
Evidence for the genomic organization of the human lambda light chain immunoglobulin joining (J) region gene segments is presented. A mouse J(lamda) probe was used in Southern hybridizations to localize joining region sequences in a cosmid clone containing the genomic cluster of six human lambda constant (C) region gene segments. The results from these hybridizations suggest the presence of at least one J gene segment upstream from each C gene segment. The regions containing the first, second, and third C gene segments were subcloned, and the areas that hybridized to mouse or human J probes were sequenced. The DNA sequences indicate that the human J(lamda)1, J(lamda)2, and J(lamda)3 gene segments have consensus nonamer and heptamer sequences, proposed to be involved in V-J joining, are capable of encoding the known amino acid sequences for the respective J peptides, and have a functional RNA splice site at the end of their coding regions. The data show that a single functional J gene segment is located 1.3 or 1.6 kb upstream of each of the C(lamda)1, C(lamda)2, and C(lamda)3 gene segments known to encode the Mcg, Kern('-)Oz('-), and Kern('-)Oz('+) isotypes, respectively. Therefore, the gene organization of this region of the human lambda locus is J1C1-J2C2-J3C3.The amino acid sequences deduced from the HuJ(lamda)2 and HuJ(lamda)3 DNA sequences are identical to mouse J(lamda)1. The DNA sequences of the J(lamda)2 and J(lamda)3 coding and flanking regions were found to be almost identical (408/409 bases). The human J(lamda)1 DNA sequence encodes a J amino acid sequence which is different from that encoded by the J(lamda)2 and J(lamda)3 gene segments, and the intron sequences immediately downstream of J(lamda)1 were only 23% homologous to the J(lamda)2 and J(lamda)3 introns (45/193 bases).The DNA sequences of J(lamda)1, J(lamda)2, and J(lamda)3 presented here establish that a single J(lamda) gene segment precedes each expressed C(lamda) gene segment. The DNA sequences of C(lamda)2 and C(lamda)3 from the Hu(lamda)9 clone differ by multiple silent mutations, and by one replacement mutation, which results in the isotypic difference between these two gene segments (Oz at position 190). The multiple differences found in the C(lamda)2 and C(lamda)3 coding sequences have ruled out the possibility that the identity of the J(lamda)2 and J(lamda)3 regions resulted from a JC duplication in this clone. Furthermore, all human germline DNA samples tested in Southern hybridizations to a fragment containing J(lamda)3 and its intron showed bands of hybridization corresponding to the J(lamda)2 and J(lamda)3 introns. The DNA sequences presented in this paper support a model for the evolution of the human (lamda) JC clusters where J1C1 and J2C2-J3C3 arose from different ancestral JC units. Models of the evolution of this locus are discussed.