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We have cloned extrachromosomal circular DNAs containing T cell receptor (TCR) delta gene segments in adult mouse thymocytes and splenocytes. We find that the frequency of circular DNA clones carrying germline delta sequences is lower than that of J alpha probe‐positive clones, possibly related to increasing 5′ distance from the most upstream J alpha segment. This suggests that the TCR alpha/delta locus is successively rearranged from within and that the delta‐containing excision products are progressively diluted out by the subsequent cell division which includes further alpha gene rearrangements. In addition, examination of delta gene excision products revealed newly identified V delta subfamilies, the reciprocal joining of two D delta elements, J delta 2 usage in thymocytes and novel sequences homologous to the human delta‐gene deleting elements.

Specificities of three mouse major histocompatibility complex (MHC) class I molecules, Kb, Db, and Ld, were analyzed by positional scanning using combinatorial peptide libraries. The result of the analysis was used to create a scoring program to predict MHC-binding peptides in proteins. The capacity of the scoring was then challenged with a number of peptides by comparing the prediction with the experimental binding. The score and the experimental binding exhibited a linear correlation but with substantial deviations of data points. Statistically, for approximately 80% of randomly chosen peptides, MHC-binding capacity could be predicted within one log concentration of peptides for a half-maximal binding. Known cytotoxic T-lymphocyte epitope peptides could be predicted, with a few exceptions. In addition, frequent findings of MHC-binding peptides with incomplete or no anchor amino acid(s) suggested a substantial bias introduced by natural antigen processing in peptide selection by MHC class I molecules.

(S,, region) indicates that the heavy chain class switch may not be mediated by stepwise linear deletion along the order of the heavy chain constant-region genes (5'-,j-y3-y1-y2b-y2a-a-3'). We propose a sister-chromatid exchange model that explains class switch-associated deletion of heavy chain genes by unequal crossing-over events between sister chromatids.Immunoglobulins are divided into five classes, IgM, IgG, IgA, IgD, and IgE, which are defined by their heavy (H) chain constituents, ,u, y, a, 8, and e, respectively. Each gene locus is itself split into two areas encoding the variable (V) and constant (C) regions (1-5). In the H chain, C region (CH) genes share a family ofV region (VH) genes. During differentiation of a given B lymphocyte, a single VH region gene (VH gene) is first expressed as a part ofthe ,t chain, and at a later stage the expressed H chain switches the C region from u chain to y or a chains without alteration of the VH-region sequence. Recent studies on the expressed H chain genes clearly demonstrate that two distinct types of DNA rearrangements take place during the differentiation of B lymphocytes (6-9). One type, termed V-J recombination, generates the complete VH gene by joining the VH-, D-, and JH-gene segments (8, 10, 11). In CH-region genes (CH genes) only one set ofthe J-region genes is proposed to be present in the 5' flanking region ofthe ,u chain gene (,u gene) (7, 9). A second type of recombination mediates the H chain class switch that associates a particular antigenbinding specificity, the completed V region, with a series of different CH regions. The rearrangement is termed S-S recombination because itjoins two switch (S) regions which are located in the 5' flanking region of each CH gene (7, 9). To switch from A to ychain, the S, region recombines with the SYregion, keeping the VH gene unaffected.It has been shown that the S-S recombination is associated with the deletion ofthe intervening DNA between the VH gene and the CH gene (12)(13)(14)(15)(16)(17)(18). Based on the order of CH-gene deletion in various myeloma cells, we have proposed an H chain gene order of 5'-g-y3-y1-y2b-'y2a-a-3'. Recent cloning experiments have directly demonstrated the order of 5'-yl-,y2b--y2a-E-a-3' (19,20).In this paper we report a detailed structure of a rearranged (expressed) yl chain gene (yl gene) cloned from a myeloma MC 101 cell. Partial nucleotide sequence determination indicates that the rearranged yl gene contains a small S. segment between the SM and S,, segments. The unique structure ofthis yl gene implicates the molecular mechanism for the class-switch (S-S) recombination. EXPERIMENTAL PROCEDURESCloning. Charon 4A (21) phage library of partial EcoRI digests of MC101 DNA was screened by using yl chain-cDNA clone pGl-6 (22) as a probe as described (23). Germline V genes were cloned from mouse embryo DNA library (provided by P. Leder, National Institutes of Health) with a V portion of the expressed yl gene (Igyl-704) as a probe. Cloning and nucleotide sequence determination of these...