THE TRANSCRIPTION FACTOR NF-KAPPA-B P50 INTERACTS WITH THE BLK GENE DURING B-CELL ACTIVATION/

The B cell-specific transcription factor Pax-5 has been shown previous ly to interact with the promoter of the blk gene in vitro, blk encodes a tyrosine kinase associated with the B cell receptor, which is expre ssed during the early but not the final stages of B cell development. To investigate whether Pax-5 regulates expression of the blk gene in v ivo during B cell development and/or activation, Pax-5a was overexpres sed in B cell lines. Increases in blk promoter activity using a chlora mphenicol acetyltransferase reporter gene system suggested a role for Pax-5a as a transcriptional activator. Subsequent site-specific mutage nesis studies showed that mutations of the Pax-5 binding site on blk s ignificantly alter promoter activity, although results suggested that other factors could bind to this region as well. Using mobility shift assays, we detected an inducible transcription factor that interacts s trongly with a sequence overlapping the Pax-5 site on the blk promoter and identified this as a homodimer of NF-kappa B/p50, a member of the NF-kappa B/Rel family of transcription factors. This factor was prese nt at high levels in lipopolysaccharide-activated normal B cells and i n plasma cell lines but either at low levels or undetectable levels in resting normal B cells or pre-B or mature B cell lines. In contrast, lipopolysaccharide induction of a pre B cell line (703/Z) induced a co mplex that contained both NF-kappa B/p50 and p65. These studies sugges t that different NF-kappa B complexes are able to interact with a sequ ence overlapping the Pax-5 site on the blk promoter and that the relat ive levels of ''bound'' factor influence levels of bit expression. Sin ce p50 homodimers and p50/p65 heterodimers of the NF-kappa B complex s hould have opposing effects on blk transcription, this could provide a mechanism to differentially regulate blk expression during B cell dev elopment and activation.