Comparative structure of two duplicated T1a class I genes (T10c and 37) of the murine H-2d MHC. Implications on the evolution of the T1a region

The class I Ag encoded in the Qa/T1a regions of the murine MHC are much less polymorphic, and usually have a more restricted tissue distribution than the classical histocompatibility class I Ag, encoded by genes located in the H-2K, D, and L loci. The isolation of a quasi-ubiquitously expressed, poorly polymorphic class I gene of the T1a region of the H-2d mouse MHC, namely gene 37 (or T18d), has been recently reported. We describe the nucleotide sequence of a closely related gene, T10c gene, the counterpart of the gene 37 in the large duplicated parts of T1a region of the BALB/c (H-2d) MHC. The T10c gene structure and sequence are very similar to those of gene 37, but T10c gene is most likely a pseudogene. In A/J mouse strain, there appears to be a single gene related to 37, which is also found expressed in a variety of tissues. We show that this gene is likely to be a chimeric one derived from T10c for its 3' part, and from a gene closely related to gene 37 for its 5' part, which potentially encodes for an unusual class I molecule composed of the first two domains. Finally, Southern blot analysis of a number of wild mice and related animals suggests that a gene closely related to the present T10c gene may be the ancestor of this subfamily of class I genes characterized by the presence of an unusual second domain.