MEMBERS OF THE NUCLEAR FACTOR KAPPA-B FAMILY TRANSACTIVATE THE MURINE C-MYB GENE

Expression of the c-myb proto-oncogene is primarily detected in normal tissue and tumor cell lines of immature hematopoietic origin, and the down-regulation of c-myb expression is associated with hematopoietic mat uration, Cell lines that represent mature, differentiated hematopoietic cell types contain 10-100 fold less c-myb mRNA than immature hematopoietic cell types, Differences in steady state c-myb mRNA levels appear to be primarily maintained by a conditional block to transcription elongation that occurs in the first intron of the gene, The block to transcription elongation has been mapped, using nuclear run-on analysis, to a region of DNA sequence that is highly conserved between mouse and man, Two sets of DNA-protein interactions, flanking the site of the block to transcription elongation, were detected that exhibited DNA-binding activities that strongly correlated with low steady-state c-myb mRNA levels. Several criteria demonstrated that members of the nuclear factor kappa B (NF-kappa B) family of transcription factors were involved in the DNA-protein interactions identified in these two sets, Surprisingly, cotransfection experiments demonstrated that coexpression of members of the NF-kappa B family, specifically p50 with p65 and p65 with c-Rel, transactivated a c-myb/chloramphenicol acetyltransferase reporter construct that contained 5'-flanking sequences, exon I, intron I, and exon II of the c-myb gene, Transactivation by these heterodimer combinations was dependent on regions of the c-myb first intron containing the NF-kappa B-binding sites, These findings suggest that NF-kappa B family members may be involved in either modifying the efficiency of transcription attenuation or acting as an enhancer-like activity to increase transcription initiation, Thus, the regulation of c-myb transcription may be quite complex, and members of the NF-kappa B family likely play an important role in this regulation.