A novel regulatory element which contributes to the regulation of quantitative, tissue-specific differences in gene expression has been found between -771 and -676 bp upstream of the major histocompatibility complex (MHC) class I gene, PD1. Molecular dissection of this element reveals the presence of two overlapping functional activities: an enhancer and a silencer. Distinct nuclear factors bind to the overlapping enhancer and silencer DNA sequence elements within the regulatory domain. The levels of factors binding the silencer DNA sequence in different cell types are inversely related to levels of class I expression; in contrast, factors binding the enhancer DNA sequence can be detected in all cells. In cultured cell lines, inhibition of protein synthesis leads to the rapid loss of silencer complexes, with a concomitant increase in both enhancer complexes and MHC class I RNA. From these data, we conclude that a labile silencer factor competes with a constitutively expressed, stable enhancer factor for overlapping DNA-binding sites; the relative abundance of the silencer factor contributes to establishing steady-state levels of MHC class I gene expression.The immune responses of all vertebrate species are mediated by molecules encoded within the major histocompatibility complex (MHC) (19). The MHC class I molecules, which serve as the targets of cellular immune responses and allograft rejection, are receptors for peptide antigens (3). These molecules are transmembrane glycoproteins consisting of a polymorphic heavy chain of 42 to 45 kDa and a nonpolymorphic light chain (P2-microglobulin) of 12 kDa.Consistent with their pivotal role in immune surveillance, the class I molecules are expressed on nearly all somatic tissues (28). However, their level of expression varies markedly among the tissues. The highest levels of cell surface expression of class I antigens occur in the lymphoid tissues: lymph node, peripheral blood lymphocytes, and spleen. Markedly lower levels are found on other somatic tissues, such as kidney and liver; there is little detectable cell surface class I antigen expression on germ line tissue or brain (28). In general, these differences reflect tissue-specific regulation of gene expression.Analysis of the 5' flanking regions of a number of class I genes has revealed the presence of a series of DNA elements capable of participating in the regulation of class I gene expression (Fig. 1A). Among these is a major enhancer, enhancer A, located at bp -180 to -170, which overlaps an interferon response element (15,20,27). A functional enhancer A is necessary for optimal expression of the downstream class I gene (13,15,18). A variety of trans-acting factors which bind to enhancer A have been identified, and their genes have been cloned (17, 21). These factors act as inducers of class I expression. Thus, KbF1 and RIIBP, both of which bind enhancer A, are associated with high levels of expression. Factors which bind specifically to the interferon response element and are induced by interferon have also b...