A 150-BASE PAIR 5'-REGION OF THE MHC CLASS-I HLA-B7 GENE IS SUFFICIENT TO DIRECT TISSUE-SPECIFIC EXPRESSION AND LOCUS-CONTROL REGION ACTIVITY - THE ALPHA-SITE DETERMINES EFFICIENT EXPRESSION AND IN-VIVO OCCUPANCY AT MULTIPLE CIS-ACTIVE SITES THROUGHOUT THIS REGION

To characterize cis- and trans-acting mechanisms that regulate MHC cla ss I transcription during development and in adult tissues, we have us ed transgenic mice to study a series of human MHC (HLA)-B7 class I gen e constructs. Previous studies identified the 5' -0.66-kb to -0.075-kb region as sufficient to direct appropriate and efficient tissue-speci fic levels of HLA-B7 RNA relative to H-2 class I. Results here show th at DNA 5' of -0.26 kb is not required for any aspect of expression. As the expression level correlated with the transgene copy number, was c omparable to H-2 or a per-gene copy basis and was independent of integ ration site, the -0.075 to -0.26-kb segment also functions as a locus control region. With this region, sequences 3' of -0.075 kb, possibly at the promoter, appear to direct the appropriate tissue distribution. Of conserved sequences in the -0.075 to -0.26-kb region, enhancer B b ox is nonessential. In contrast, in vivo ''footprinting'' implicated r egion I/enhancer A/NF-kappa B, IFN consensus/response sequence, and al pha in class I regulation as they are ''occupied'' in a tissue-specifi c pattern that correlates with expression. Mutation of alpha leads to decreased expression and loss of occupancy not only at alpha but also at region I/enhancer A/NF-kappa B and IFN consensus/response sequence. Thus, site alpha is an essential class I regulatory element, the domi nant function of which is to mediate tissue-specific occupancy at mult iple adjacent cis-active sites, possibly by facilitating stable synerg istic interactions between factors at these distinct elements.