PHYSIOLOGICAL TARGET OF THE AIR-1 TRANSACTIVATOR REVEALED BY STABLE TRANSFECTION ASSAY

RJ 2.2.5 is a human B cell mutant, derived from Raji cells, which has lost expression of major histocompatibility complex (MHC) class II gen es because of a defect in the AIR1 locus function. The MHC class II-po sitive phenotype can be restored by introducing an active AIR1 locus o r its mouse equivalent, Air-1. An example of the latter is the H4 cell hybrid, derived by somatic cell fusion between RJ 2.2.5 and mouse cla ss II-positive spleen cells. H4 contains a single mouse chromosome, au tosome 16, in which the Air-1 locus maps, and an entire RJ 2.2.5-deriv ed genome. In the present study we show that the physiologic target of the Air-1 locus product is contained within a limited HLA-DRA promote r sequence of 300 base pairs, encompassing the crucial Y, X, and W cis -acting elements. A plasmid construct, pDRA300neo, containing the HLA- DRA promoter sequence which drives the expression of the neomycin resi stance gene, has been stably integrated in the genome of the H4 hybrid . Transfectants selected in the presence of G418 retain mouse chromoso me 16 and express the DR genes. On the other hand, transfectants grown in a non-selective medium segregate mouse chromosome 16; this is acco mpanied by a loss of DRA gene expression and G418 resistance, although pDRA300neo is still integrated in the genome. These results offer sco pe for using this experimental model to clone the Air-1 gene in a stra ightforward way.