Single base pair substitutions within the HLA-DRA gene promoter separate the functions of the X1 and X2 boxes.

The class II MHC genes are expressed on the surfaces of B cells, activated T cells, and macrophages and may be induced in other cell types by IFN-gamma. The control of class II gene expression has been shown to be mediated by a series of conserved cis-acting sequences (W, X1, X2, and Y boxes) located immediately 5' to the genes. Although these sequences are conserved, the bp that are important for transcriptional regulation have yet to be identified. To address this issue with regard to the MHC gene HLA-DRA, a series of single bp substitutions spanning the conserved upstream sequences was created and analyzed for their effects on transcription in both B cells and IFN-gamma-treated fibroblasts. In addition, the effects of X1 and X2 box mutations on DNA/protein interactions were examined and compared to the transcriptional data. The results of these studies show that each of the conserved elements participate in maximal expression in B cells and that W, X1, and X2 boxes are important for IFN-gamma induction and expression in fibroblasts. Interestingly, some of the bp changes that altered B cell expression did not alter expression and IFN-gamma induction in fibroblasts, suggesting that different or altered factors control the expression of these genes in the different cell types. Mutant templates designed to eliminate the binding of X1- and X2-specific DNA binding proteins in vivo suggest that these elements and their factors may interact to promote transcription.