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We have characterized immunoglobulin switch circular DNA in mice infected with the nematode parasite Nippostrongylus brasiiensis. Two kinds of circular DNA were identified in the lymph nodes as excision products of switch recombination of immunoglobulin heavy-chain constant region (CH) genes. One is a recombinant between C., and C,1 (yl circle), and the other is a recombinant between Cy1 and C, (E circle). In the E circle, a short piece of switch ,u (S,,) sequence was inserted between SE, and Sy7 sequences. The inserted S,. sequence could be a trace of the preceding switch from CA to C.,l. These findings indicate that parasitic infection can induce class switch recombinations in a successive manner, first from CA, to Cy1, and then from Cad1 to C,.

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In order to isolate cDNA clones for DNA-binding components of the V-(D)-J recombinase, phage libraries from a pte-B-cell line were screened with a radiolabeled probe containing recombination signal sequences (RSS). Among prospective clones, cDNA T160 was analyzed further. It produced a protein of 80.6 kDa which bound to DNA containing RSS but not to DNA in which the RSS had been mutated. A search of a data base revealed that the T160 protein has significant sequence homology (56%) to the nonhistone chromosomal protein HMG1 within the C-terminal region of 80 amino acids. DNA-binding analysis with truncated proteins showed that the HMG homology region is responsible for DNA binding. Using restriction fragment length polymorphisms, the T160 gene was mapped at the proximal end of mouse chromosome 2. Evidence was obtained for genetic linkage between the T160 gene and the recombination activator genes RAG-1 and RAG-2.Somatic-DNA recombination activates and diversifies variable-region genes of antigen receptors (29,51). This DNA rearrangement, known as V-(D)-J joining, occurs between two pairs of recombination signal sequences (RSS), i.e., a heptamer (5'-CACAGTG-3') and a nonamer (5'-GGTTTTTGT-3'), when one pair is separated by a 12-bp spacer and the other is separated by a 23-bp spacer (9,12,35,(42)(43)(44). Substrate requirements and the effects of base substitutions have been studied in detail for recombination signal sequences (2,18). For the enzymatic machinery, several nuclear proteins have been reported as having possible endonucleolytic activities (11,19,25) or as being DNA-binding components of the putative V-(D)-J recombinase (1,16,17,32,34).Two chromosomal locations have been reported for recombination functions associated with V-(D)-J joining. A defect in DNA recombination has been observed in the scid mouse, CB-17 scid (5), in which V-(D)-J joining is apparently impaired in both B and T cells. Extensive DNA deletions are found at recombination junctions in the scid animal (33, 39). The CB-17 scid gene has been mapped to mouse chromosome 16 (6). Recently, two genes that can activate V-(D)-J joining were isolated by introducing chromosomal DNA fragments into the fibroblast cell line NIH 3T3 (38,46,47 protein is much larger than other HMG members, the hydrophilic region of T160 has a strong homology to HMG1. In order to study whether the T160 gene has a genetic linkage to scid or rag, the chromosomal location for T160 was determined. Using restriction fragment length polymorphism segregation analysis, genetic linkage was established for T160 and RAG genes on the proximal end of chromosome 2. In this report, we characterize the T160 protein and discuss its possible role in V-(D)-J recombination.MATERIALS AND METHODS cDNA library. Total RNA was prepared from a pre-B-cell line, 38B9 (4), according to the manufacturer's protocol (Amersham) and passed through an oligo(dT)-cellulose column three times to obtain poly(A)+ RNA. cDNA was synthesized by the method of Gubler and Hoffman (15) using oligo(dT) and random he...

Circular DNA resulting from V gene replacement was studied with an A‐MuLV transformed cell line containing ablts. This cell line undergoes V gene replacement at elevated temperatures in the immunoglobulin (Ig) heavy chain (H) gene. Examination of circular DNA revealed that a heptamer‐related sequence (TACTGTG) within the coding region of VDJ was joined to the recombination signal sequence (RSS) of a germline VH segment. This provides direct evidence for a intramolecular DNA deletion mechanism for V gene replacement. In the pre‐B cell line as well as in in vivo lymphocytes, unusual circular DNAs were found which were structurally similar to the V gene replacement circles. They represented excision products of the deletion type recombination between one complete RSS and a heptamer‐like sequence in the Ig H region.