IN-VIVO MODIFICATION OF MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II DRAPROMOTER OCCUPANCY MEDIATED BY THE AIR-1 TRANSACTIVATOR

RJ 2.2.5 is a human B cell mutant derived from the Burkitt lymphoma Ra ji cell, which is defective in the AIR-1 locus function. This locus en codes a transcriptional trans-activator required for the constitutive expression of major histocompatibility complex (MHC) class II genes. H ere we show, by in vivo DNase I footprinting, that the AIR-1 locus def ect correlates with changes in the DRA promoter occupancy. Interesting ly, reexpression of human MHC class II genes in RJ 2.2.5 x mouse splee n cell hybrids is associated with partial reversion of DRA promoter oc cupancy to the Raji pattern. DRA promoter occupancy in other class II- negative B cell lines, derived from patients with bare lymphocyte synd rome, is drastically different from the one observed in RJ 2.2.5 and R aji cells. Moreover, the use of the DNase I as an in vivo footprinting agent reveals that the patients' cell lines do not display a complete ly ''bare promoter'' as previously reported using dimethyl sulfate as the footprinting agent. Thus, the use of DNase I allowed us, for the f irst time, to correlate the AIR-1 locus defect with class II promoter occupancy alterations and distinguish these alterations from the ones observed in phenotypically similar but genetically distinct MHC class II-negative cells.