TRANSCRIPTION OF THE BLK GENE IN HUMAN B-LYMPHOCYTES IS CONTROLLED BY2 PROMOTERS

Genomic DNA containing the first exon and 5'-flanking region of the hu man protein tyrosine kinase, blk, was isolated. Sequence analysis iden tified a TG repeat element in this region with enhancer activity, but no TATA or CCAAT sequences were found. Two blk transcripts of 2.2 and 2.5 kilobases were identified in various B-cell lines by Northern blot analyses, and primer extension experiments demonstrated two clusters of multiple transcription start sites. Subsequent promoter analyses by transient transfection assays with a reporter gene identified two pro moter elements in the human blk gene. Promoter P1 contains sequences t hat have been shown to regulate the expression of immunoglobulin genes and promoter P2 contains elements that are highly conserved in the pr omoter of major histocompatibility complex class II genes, as well as a B-cell-specific activator protein- (BSAP) binding site. Electrophore tic mobility shift assays demonstrated that the binding of a protein t o the BSAP-binding site was correlated with the presence of the 2.5-ki lobase blk transcript. These data suggest that the two human blk RNAs arise from the transcription of the blk gene by two distinct promoters and that these promoters may be subject to regulation by different tr ans-acting factors.