Recently it has been suggested that a short segment of the DNA sequence of the Ia beta gene in the mutant B6. C-H-2bm12 (bm12) is derived from the Eb beta gene by a gene-conversion-like mechanism, and that the same segment is also seen in the Ek beta gene. To obtain serological evidence for this idea, we produced an antibody against the “new” determination on the I-Abm12 molecule by immunizing A.BY mice with bm 12 cells. After absorption with B6 lymphocytes to remove antibodies against background antigens, the antisera lysed bm12 cells. Typical Ia peaks were obtained by immunoprecipitation. The absorbed antiserum reacted with B10.A (Ek β), B10.A(5R) (Eb β) and B10.S(9R) (ES β), but not B10. The unabsorbed antiserum is specific for Ia when tested on A background mice. The antiserum lysed spleen cells of Ik strains (A/Sn, A.AL, A.TBR1, and A.TFR1) but not Ak strain, A.TBR13 (KsAkEbSbDb), confirming the presence of antibodies aganist the I-Ek molecule. This antiserum also lysed the cells from (A.BYxA.TFR5)F1, which expresses the transcomplementing Es βEK α molecule and from the (A.SWxA. TFR5)F1 which expresses the transcomplementing EsβEkα molecule. These data are consistent with DNA sequence analyses, and show the existence of a determinant (Ia.51) generated in the bm 12 mutant by a gene-conversion-like event that is also present in the I-Ek, I-Eb, and I-Es beta polypeptide chains.
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