Genomic sequence and organization of rabbit Tcrb constant region genes

We previously reported that some rabbits have three different copies of T-cell receptor b (Tcrb) constant region genes unlike man, mice, and rats who generally have two copies. Two of these C beta genes were found on an approximately 14 kilobase (kb) and one on an approximately 6 kb Eco RI fragment. The gene on the 6 kb fragment is of beta 2 type. A previously described portion of the 14 kb fragment appeared to have sequences characteristic of C beta 1. We have now shown that the 6 kb fragment is adjacent to and 3' of the 14 kb fragment. Furthermore, the second linked sequence of C beta gene present on the 14 kb fragment resembles to a large extent the C beta 2 gene present on the 6 kb fragment. Moreover, this second C beta gene has a 5' cluster of J beta sequences resembling J beta 2 of other species. However, exon 4 and the 3' untranslated region (3'UT) are of the beta 1 type. Mapping studies using Southern analyses of both genomic DNA and the 14 kb clone have identified another cluster of J beta 2 sequences 5' of the third tandem C beta 2 gene present on the 6 kb Eco RI fragment. Thus, the second gene on the 14 kb fragment appears to be a chimeric genomic Tcrb gene that may have arisen by an unequal crossing-over event analogous to that which may have deleted C beta 1, D beta 2, and J beta 2 in NZW mice.