IDENTIFICATION OF A TRANSCRIPTION FACTOR THAT BINDS TO THE S-BOX OF THE I-A-BETA GENE OF THE MAJOR HISTOCOMPATIBILITY COMPLEX

Class II genes of the MHC show a striking homology upstream of the tra nscription start site that is composed of three conserved sequences (S , X and Y boxes, each separated by 15-20 bp). The presence of the S-bo x sequence in the mouse MHC class II gene I-A beta was examined for it s influence on the expression of this gene. Deletion or mutation of th e S box decreased the induction of chloramphenicol acetyltransferase ( CAT) activity in B lymphocytes by 32%. In macrophages, deletion or mut ation of the S box abolished interferon-gamma (IFN-gamma) inducibility of CAT activity. Using a gel-retardation assay, we have identified a nuclear factor whose binding site overlaps the 7-mer conserved sequenc e of the S box. This factor is present in lymphocytes, macrophages, ma stocytes and fibroblasts. Surprisingly, binding of this nuclear factor to DNA was induced by IFN-gamma in bone-marrow-derived macrophages, b ut not in macrophage-like cell lines. The binding site for this factor was defined by DNase I footprinting and partially purified by using a n affinity column containing double-stranded oligonucleotides containi ng a sequence of the S box. A prominent protein of 43 kDa was found th at bound specifically to the S-box sequence.