Polymorphism at the mouse Thb locus.

Mouse ThB is a 15,000 M(r) glycosyl phosphatidyl inositol anchored cell surface glycoprotein that shares amino acid homology with Ly-6 molecules; the gene is closely linked to Ly-6 on chromosome 15. The Thb locus has two alleles, Thbh and Thbl, which control the level of expression of ThB molecules on thymocytes (as shown herein) and on splenic B cells, and is therefore different from the usual polymorphisms of other Ly loci which give an all or none serological reaction. The reason for the expression polymorphism is unknown and could include a different protein structure in ThB molecules, altered glycosylation, or differences in transcriptional control. To determine the reason for the differences in expression, we examined the RNA (cDNA) sequence of Thbh and Thbl alleles: there was complete nucleotide identity in the cDNA sequence in both ThB(high) (C57BL/6)- and ThB(low) (BALB/c)-expressing strains; the RNA and protein sequences would therefore be identical. In addition to the difference in the amount of cell surface glycoprotein, there was also a difference in the level of ThB mRNA in ThB(high) and ThB(low) strains; thus the differences in ThB expression are likely to be due to different rates of transcription of the two alleles.