Identification of a novel human BCL-X promoter and exon

BCL-X-L is a key anti-apoptotic BCL-2 family protein that is widely expressed in human cancer cells and is induced in response to diverse survival signals. The translation initiation codon for BCL-X-L is located in BCL-X exon II and previous analyses have indicated that BCL-X-L RNAs initiate close to the start of exon IT or additionally contain a non-coding first exon (exon IA) spliced to exon TI, Using 5' RACE we have now identified a novel BCL-X non-coding exon (exon IB) which is spliced directly to exon IJ[ in place of exon IA, Exon IB-containing RNAs encoded BCL-X-L and were detected in non-malignant lymphocytes and lymphoma cells from lymph node biopsies and mere expressed at significant levels in cell lines derived from ovarian, colon and breast cancers. We identified two TATA-hox sequences upstream of exon TB and demonstrated that surrounding genomic sequences contained strong promoter activity in lymphoma cells (approximately 300-fold active relative to controls). We have therefore identified a powerful new BCL-X promoter and a novel exon that contributes to BCL-X-L expression.