Identification of a c-myb attenuator-binding factor

Expression of the c-myb proto-oncogene is developmentally regulated at the level of transcription elongation. In pre-B cells, complete c-myb transcripts are produced, whereas transcripts are attenuated near or within a 300-base pair (bp) interval of the first c-myb intron in mature cells. Hypothesizing that transcription attenuation results from a protein complex that physically impedes the progress of RNA polymerase II through the intron, we used electrophoretic mobility shift assays (EMSA) to search for DNA-binding activities that correlated with downregulation of c-myb transcription. We identified a stage-specific DNA binding activity, termed ABF, present in mature B cells but not in pre-B cells. ABF binds to a 15-bp DNA element located within a 300-bp BstEII-XbaI fragment. DMSO-treatment of murine erythroleukemia cells results in rapid downregulation of c-myb transcription and upregulation of ABF DNA binding activity. Thus, ABF binding activity correlates with downregulation of c-myb transcription in two systems. Preliminary biochemical characterization of ABF from mature B cells demonstrates that its primary DNA-binding component is a 64-kDa-protein. We hypothesize that this factor may represent a member of the transcriptional attenuation complex.