THE LOCUS-SPECIFIC ENHANCER ACTIVITY OF THE CLASS-I MAJOR HISTOCOMPATIBILITY COMPLEX INTERFERON-RESPONSIVE ELEMENT IS ASSOCIATED WITH A GAMMA-INTERFERON (IFN)-INDUCIBLE FACTOR DISTINCT FROM STAT1-ALPHA, P48, AND IFN REGULATORY FACTOR-I

Recent analyses of the upstream regulatory regions of the class I majo r histocompatibility complex genes in higher primates provided a gener alized structural basis for the differential expression of A- and B-lo cus gene products in response to specific physiological stimulus, Amon g the regulatory sequences that differ between the loci is the interfe ron-responsive element (IRE), While the B-IRE is conserved, the A-IREs have species-specific sequence variation, We previously demonstrated that the B-IRE was an interferon (IFN)-inducible enhancer, whereas non e of the A-IREs were functional, In the present study, we examined the biochemical basis for the enhancer activity of the conserved B-IRE an d found that this may be attributed to a novel gamma-IFN-inducible fac tor, This factor accumulated in nuclei of cells within minutes of expo sure to gamma-IFN. Its appearance was independent of de novo protein s ynthesis, However, it was not detected in nuclei of cells treated with herbimycin A, suggesting that its appearance depends on a protein kin ase activation pathway, Supershift assays indicated that it was distin ct from STAT1 alpha, IFN regulatory factor-1, and p48, transcription f actors known to bind IRE-like sequences found in regulatory regions of many nonmajor histocompatibility complex gamma-IFN-responsive genes, Competition assays show that this novel factor bound B-IRE with relati vely high affinity, about 100-fold more than that for the A-IRE sequen ce, This factor was also present in STAT1 alpha and p48 somatic mutant s that also exhibited B-IRE enhancer activity in reporter gene bioassa ys in a manner similar to those seen with wild type cells, These obser vations indicate the existence of a novel gamma-IFN-dependent transcri ptional activation pathway that correlates with the differential enhan cer activity of the HLA-B IRE.