Structure and Evolution of the Hla Class-ii Sx-beta Gene

The class II major histocompatibility complex antigens are cell-surface heterodimers consisting of an a and a β chain. Cosmid cloning has shown that the three families of clas II antigens, DR, DQ, and DP, are encoded within the HLA-D region of chromosome 6 as a series of discrete gene clusters. The DP cluster contains two pairs of a and β genes, one of which encodes the biochemically-defined DP antigen. In order to assess whether the other two genes, SXa and SXβ, are also expressed, potential coding regions have been subcloned and sequenced. The SX3β gene is shown to contain region closely homologous to all six exons of DP β. A 1 bp deletion in the β2 exon, also observed for the SX4β allele, causes a translation frameshift, suggesting that SX β is a pseudogene. However, all the other exons, as well as their splice sites and the putative promoter region, appear to be intact.