A molecular basis for species differences in Thy-1 expression patterns.

Thy-1 is a membrane glycoprotein that displays species-specific differences in its pattern of expression. Although it is expressed on thymocytes and splenocytes in mice, it is only expressed on thymocytes in rats. Based on previous studies suggesting that the third intron of the mouse Thy-1 gene is required for its expression in thymocytes, in vivo footprinting analysis was performed on the third introns of both the mouse and rat Thy-1 genes, and led to the identification of homologous 36 bp "footprinted" regions. The mouse 36 bp region was found to be capable of specifically binding an Ets-1-like nuclear factor present in both mouse thymocytes and splenocytes. In contrast, the homologous 36 bp region of the rat which differs from the mouse 36 bp region by three nucleotides resulting in the loss of the Ets-1 binding site, is unable to bind a similar Ets-1-like factor present in rat thymocytes. Instead, this region of the rat third intron binds another nuclear factor which is present in rat thymocytes but not splenocytes. These observations suggest that the differential expression of the mouse and rat Thy-1 genes in thymocytes and splenocytes is the result of differential expression of nuclear factors that bind to this 36 bp region.