An HLA-DR alpha promoter DNA-binding protein is expressed ubiquitously and maps to human chromosomes 22 and 5.

The class II major histocompatibility complex antigens are a family of integral membrane proteins whose expression is tissue-specific and developmentally regulated. Three consensus sequences, X1, X2, and Y, separated by an interspace element, is found upstream from all class II genes. Deletion of each of these sequences eliminates expression of class II genes in vitro or in transgenic mice. Here we further characterize the expression of a cDNA encoding a DNA binding protein (human X-box binding protein, hXBP-1) which, like the proteins in whole nuclear extract, recognizes both the X2 promoter element of the human DR alpha and DP beta and mouse A alpha genes. The hXBP-1 cDNA hybridizes to human RNA species of approximately 2.2 kilobases (kb) and 1.6 kb, which are expressed in class II negative as well as class II positive cells. hXBP-1 transcripts are present in several class II deficient mutant B cell lines, although in one such line, 6.1.6, levels were somewhat reduced. Chromosome mapping studies demonstrate that hXBP-1 arises from a small gene family, two of whose members map to human chromosomes 5 and 22. Taken together, these data suggest a high degree of complexity in the transcriptional control of the class II gene family.