THE TRANSCRIPTION FACTOR B-CELL-SPECIFIC ACTIVATOR PROTEIN IS NOT INVOLVED IN THE IL-4-INDUCED ACTIVATION OF THE HUMAN IGE GERMLINE PROMOTER

Transcriptional activity of the human IgE germline gene is a prerequis ite for a subsequent deletional rearrangement of the Ig heavy-chain lo cus, the hallmark of isotype switching to IgE. The B-cell-specific tra nscription factor B cell-specific activator protein (BSAP) was describ ed for being critically involved in the IL-4 up-regulation of the muri ne IgE germline gene. Our study was initiated to evaluate the regulato ry role of BSAP in the human IgE germline promoter. It is shown that B SAP binds to a DNA element located immediately upstream of the most 5' transcriptional start site. The authenticity of BSAP was determined b y electrophoretic mobility shift assays in which oligonucleotides corr esponding to published BSAP binding sites efficiently competed for bin ding to the novel identified sequence. In addition, recombinant purifi ed BSAP protein bound this motif and comigrated with the band seen wit h nuclear extracts. Finally, a polyclonal anti-BSAP antiserum specific ally prevented interaction of the protein with its DNA recognition seq uence. The affinity of BSAP for its recognition sequence was low compa red with the sites identified in the CD19, the blk gene, and an LR1 tr anscription factor binding sequence located in the Ig gamma(1) switch region. Reporter gene constructs in which binding of BSAP was abolishe d by site-directed mutagenesis responded to IL-4 stimulation better th an the wild-type construct in both cell lines tested. In addition, the basal activity of the mutated promoter did not change significantly d espite the close proximity of the BSAP motif to the transcriptional st art site. It is concluded that BSAP plays no direct regulatory role in the cytokine-induced response of the human IgE germline promoter.