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c-Myb Repression of c- erbB-2 Transcription by Direct Binding to the c- erbB-2 Promoter (*)

The c- myb proto-oncogene product (c-Myb) is a transcriptional activator that can bind to the specific DNA sequences. Although c-Myb also represses an artificial promoter containing the Myb binding sites, natural target genes transcriptionally repressed by c-Myb have not been identified. We have found that the human c- erbB-2 promoter activity is repressed by c-Myb or B-Myb in a chloramphenicol acetyltransferase co-transfection assay. Domain analyses of c-Myb suggested that Myb represses the c- erbB-2 promoter activity by competing with positive regulators of the c- erbB-2 promoter. In in vitro transcription assays, Myb proteins containing only the DNA binding domain could repress c- erbB-2 promoter activity. Two Myb binding sites in the c- erbB-2 promoter were critical for transcriptional repression by c-Myb. One of the two Myb binding sites overlaps the TATA box, and DNase I footprint analyses indicated that c-Myb can compete with TFIID. These results suggest that Myb-induced trans-repression of the c- erbB-2 promoter partly involves competition between Myb and TFIID.